Some of the review:

this game sux
i cant get past the jelly king level
3/10

I like how these people say they hate the game, yet hey have less than 2 hours on it, also half of them "recommend the game" yet they give a negative review (must be because of the newly implemented system where all recommendations became reviews) but still. Half of them haven't even gotten passed .2 hours XD

Here.
I played the game for 300+ hours and made a review. Maybe you guys can take this one more seriously than the others.
Is this what you wanted?

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Not recommended

Having played the game for over 300 hours, seeing many iterations of the game, and seeing the general direction of where the game is headed, I honestly can't recommend it for most players.
There's a lot of problems with the way the game is run and how it is being developed that are more than a little concerning. Over the course of the two years I've played this game, I've gone from having a lot of hope for it and being really excited to see how it develops, to being sorely disappointed with the quality of the updates and actually feeling sad for all the squandered potential the game had.

Let's get into details now shall we?

Positives:
- Gameplay
Pretty unique in its gameplay. Haven't seen any other MMO that plays quite like it. A good take on translating the top-down Legend of Zelda style play and turning it into an MMO. It's easy to understand and play, with some advanced techniques to pick up along the way.
The randomly generated levels also add a bit of replayability to the game.
- Sound
The music for the game is pretty good. Though generally upbeat and calm, it can get hectic or atmospheric when it needs to.
The sound effects are also pretty decent and add a lot of character to the colorful cast of enemies you'll find yourself beating down.
- Art
The absolute biggest positive of the game in my opinion. Virtually every art asset is well designed and rendered in a very unique and eyecatching style. They're also very well animated to boot!
Granted, there are some exceptions where lazy design or lackluster work sticks out (an example being a certain line of upgradeable helmets only has 2 textures between all 6 of them), but overall the art is what stands out the most to me.
- Community
Honestly one of the nicest MMO communities I've seen. There's hardly any hostilities towards other players and most players are genuinely helpful towards one another. The glaring exception being the PvP minigame, Lockdown.

Negatives:
- Updates
One of the most disappointing things about the game is the general lack of content brought by updates. It is understandable that updates aren't frequent when you realize how small the OOO dev team is, but the amount of content released over the course of the two years I've played is pitifully small. A lot of the "new" content is just rehashed older content to pad out the game or one-time bosses, enemies, and set pieces put into single Missions while the rest of the gameworld (which is played VASTLY more than most single Missions) is in sore need of more variety.
And that's not even getting into the annoying cash-grab updates dedicated solely to costumes.
- Performance
Being an MMO built on Java, there's definitely going to be performance issues unless the game is perfectly optimized (which it isn't). An abundance of entities on the screen can slow the game to a crawl no matter the quality, which wouldn't be a problem if there weren't also an insane abundance of levels that use huge hordes of enemies and particle effects. The game lags a lot and that's not even getting into the extremely small number of servers causing problems for those that can't get a decent connection.
Then there's also an inherent problem with the game running on Macs which further harms performance. Not as pronounced a problem, but definitely noticeable.
- Balance
One of the biggest breaking points for me. The balance of the arsenal of weapons and armor is abyssmal. Some weapons are inexplicably far more powerful and useful than others. Others are so painfully useless or weak that it's a wonder if they get used at all.
Example: Any armor that "emphasizes" defense doesn't offer much more defense than other armors that also offer major attack bonuses. Even then, most of the defensive sets only allow you to take a single extra hit or maybe two incomparison to the offensive sets. Essentially, any armor set that doesn't boost your attack power in some way is obsolete to those that do.
It's been this way for YEARS, with the community clamoring for balance and suggesting ways to balance things out. Some balance changes were even promised for underpowered or broken weapons, but after more than a year on some of them, no changes have been made.
It completely baffles me why the devs can't simply change numbers around on the equipment, but it apparently is too tough for them to do so.
Speaking of broken balance...
- Lockdown
The broken balance certainly doesn't help things in Spiral Knight's PvP mini-game. Aside from equipment making it hard to take seriously, it's a PvP game and generates all the toxicity that a PvP crowd can. Unless you're really into PvP, it's best to steer clear from this unbalanced mess.
- Grinding
Being a Free-to-Play MMO, grinding is to be expected. However, with the limited number of randomly generated areas and how little moneyand experience some areas offer in comparison to the boss levels, it gets incredibly dull, incredibly fast.
And then there's the issue of the Forge, a mechanic that slows down a player from levelling their equipment. Aside from needing the game's equivalent to experience, you also need a rather big number of rare items for a CHANCE to bring your euipment to the next level unless you use a LARGE number of said rare items to get it that chance to 100%. Then there's the fact that you can't access the Forge from everywhere in the game, which creates a disconnect between gaining a level and that rewarding feeling of leveling up.
- Difficulty
I don't quite agree with OOO's idea of difficulty being to add more health and damage onto monsters or to throw so many monsters at the players that damage is nigh unavoidable. It just reinforces the superiority of the offensive sets of equipment since they can deal with both sitautions faster and more efficiently. More diverse groupings of enemies or improving the AI along with the general power given to the players is the more fulfilling and strategic route to go, in my opinion. Giving the mosters more health and damage just makes battles long, grindy, and extremely frustrating should they touch you.
- Revive system
This used to be a big plus for the game until the mechanic was completely overhauled earler this year.
Before, players could pick up any allies that got incapacitated at the cost of half of their health. It encouraged teamplay and helping each other out, while also making most players cautious with their health since it could save a teammate as well.
Now, you can't offer your health to revive your team. It instead relies on a rare item that you can either get from treasure drops or purchase with your own money. If you want to revive a teammate it's going to cost you one of your own revive items and to even do so you have to open a menu and select to revive said teammate. It takes a lot more time and is never even mentioned as an option in the game as opposed to the old method where all you had to do was stand near your teammate, see the revive icon, and click.
This change really broke the teamplay aspect of the game for me. Instead of your team making daring rescues to revive one another, now you have your team lying on the floor begging for a revive because they don't want to use their own.
- End game
The end game has been stagnant for more than a year now. Once you have your 5 Star equipment set and reach the end of the Missions there honestly isn't much to do unless you have money to pay for the Shadow Lairs. Even then, the Shadow Lairs are seriously just rehashed boss levels with hordes more enemies to make it more "difficult." What's worse is that you have to pay an exorbitant amount of Energy/real money to even enter one of them ONCE. All for useless, gimmicky pieces of armor.

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And that's my review for the game. Feel free to give it a look (it is free afterall), but be warned that there are a LOT of problems with the game.
Personally, I'd recommend trying out another free MMO like Warframe or maybe Vindictus.

I never said that Warframe was the best thing ever or anything like that. Just listed it as a possible alternative for a F2P MMO to play. I honestly can't even play it due to being on a Mac (hence why I know of the Mac problems on Spiral Knights), but would honestly love to give it a try.
My brother used to play Vindictus and that's why I suggested it as well. It certainly has its own set of problems, especially since it makes good friends with the RNG. But it's a pretty cool looking game with heavy action oriented combat.

Also, those two games, especially Warframe since it's fairly new, are updated or have been updated with more content than SK. Vindictus is a long running game that has a fairly long story campaign, so even though the quality of its updates have dropped, there is a ton of content to play before you hit endgame.
I'd recommend them just because people would have more fun for a far longer period of time than SK. With the free elevators and all the quest rewards, most players can reach Vanguard within a week or two. After that, there is seriously NOTHING to do. The end-game has been painfully stagnant for YEARS. OOO has done little to keep their Vanguards from up and leaving aside from keeping some of them sated with the Guild Halls. Instead, all they do is try to pander to new crowds and leave Vanguards with very limited end-game content to play. There is ONE legitimate T3 boss that is free to fight. ONE. All the others are locked behind an absurdly expensive paywall and offer nothing but gimmicky armor sets and four scraps of lore.
I'd much rather play a game that is expanding its end-game constantly than one that ignores its established playerbase in favor of the wallets of a bunch of new kids.

And your Warframe in a nutshell sounds much nicer than how I'd describe SK.
Buy armor and weapons. Grind them. Grind the levels. Grind the bosses. Get bored. Buy more. Grind those. Realize there's just no more content to play. Wait for end-game content. Wait for end-game content. Wait for end-game content. Leave.

Once again, I haven't played Warframe, so I can't comment on its "polish" or playable content with firsthand knowledge. 200+ Missions with 10 different Mission types in randomly generated levels seems like decent and varied content though. :/

Still, regardless of how much content and polish it may or may not have currently, it seems to be doing fairly well and getting updates with sexy new Frames, weapons, and balance changes. The devs also talk and actually take suggestions from the community with the "list" thing they have going.
It's also gaining significant notoriety, especially considering that it's moving onto the next-gen consoles as well.
I'd honestly be surprised if they dropped the ball so hard after it gets out of beta that it fails horribly in some way.

But even taking Warframe out of the picture, there are sure to be a lot of F2P MMO games to play that get meaty updates, have a better sense of balance, and a more expansive end-game. Granted, there are just as many that can be terrible at those things... like SK! :D

Yes.
Yes I would.
I LOVE video games and could probably talk for days about them. I do research when looking into games that I want to put time into. Granted, I can't play a lot of these games though since owning every console, OS, handheld, and game is pretty expensive, but I try to watch enough Let's Plays and gameplay videos, read reviews from players, and try to get firsthand perspectives from acquaintances that I can get a decent grasp of whether something is a decent game or not.
If someone asks me whether or not to play a game, I'll try to get them informed as possible about it even if I have to inform myself first. (I'll let them know if I've played it or not though. I just honestly didn't have room on my Steam Review since I seriously used every character possible. It also wasn't the point of the review or my post whether I played Warframe or not.)

Even then, from everything that I've seen, read, and heard, Warframe is worth looking into. There are enough detailed reviews that suggest it's a sound choice to put a little time into. It's also got enough support and popularity that it's hard to see it going downhill soon.
Besides, if they like it, they can stick with it.
If they don't, even though they lost some time, they at least didn't lose any money trying it.

I can't say the same for SK.
Even though it's free to play and as nice as it looks, there are too many problems within the game and with the devs that I can't seriously recommend it without a lot of warning. I don't want them putting time into something that honestly doesn't look like it's going anywhere. The only reason why I stick around is because of how attached I am to it and that I still think the art design is good.
Back when I really liked the game and tried to play daily, I would try to get others into it, especially my brothers and sister. They thought it looked cute and was fun, but quickly grew bored of it. And I mean REALLY quickly. The furthest any of the them made it was 2* before they went back to their own games. Hell, I grew bored with it and had to stop playing it for months at a time before coming back. Multiple times. I'm still surprised I made it to Vanguard with how little I play.

Can you honestly recommend someone to play Spiral Knights?
With the immense amount of grinding?
The stagnant end-game?
The utter lack of balance?

But this thread isn't even talking about Warframe? It's about SK reviews.
I might not know about the inner workings of Warframe, but after 300 hours, reading enough of Zeddy's science threads, and watching how OOO handles their own game over the course of two years, I think I know enough about Spiral Knights to write a review for it.

Also, if you've heard about a good game (say Journey, Shadow of the Colossus, Minecraft, Terraria, Starbound, Thomas Was Alone, Bastion, etc.) that you just haven't played personally, but have had enough of your friends play or heard amazing reviews for it, would you recommend them?
There are thousands of great games out there and the only people I expect to try play them all are gaming critics. There's just not enough time in anyone's lifetime to experience all the content that's out there.

So if I have to make a choice between a game that I've played and realize is a poor experience or a game that I haven't played but have heard good things about from multiple sources (mostly friends who used to play SK, mind you), which one should I recommend?