I'm a Steam user, and I was able to install the preview just fine through the Steam URL (note that the Steam install URLs do work on Chrome nowadays, though I 'am' on the dev version). Of note, Spiral Knights crashes for me if I have the Steam overlay enabled, and that still happens on the preview, so the issue might be on my end.
First thoughts so far... Very positive. I'm a Gunner, and seeing all these gun types and damage type combinations really opens up gunning a lot more. No longer am I restricted to certain types of guns if I want a particular kind of damage. It feels so much more flexible, allowing me to custom-tailor my loadout to exactly how I want to play (even if that means equipping all Polaris-type weapons XD).
EDIT: I just noticed that players retain full (or near full) walking speed when charging pretty much all new guns. This is a welcome change. As a Gunner, I rarely, if ever, charge my guns except on a party button, so this might encourage more charging. For most guns, though, guns are easier to use and do more damage with regular fire, but this change is still convenient.
(The projectiles are faster now, right? I'm pretty sure they are, but there's no regular Polaris for me to buy to compare to...)
So we now have a Shock Elemental (Polaris), Fire Elemental (Wildfire), Freeze Shadow (Permafroster), and Normal (Supernova). I am concerned about the Wildfire, because Shock is only marginally useful at fighting enemies. It'll do damage, but unless it's in a group of bunched enemies, Fire does more, suggesting that Wildfire may virtually obsolete Polaris, except perhaps for Fiends and FSC.
Of course, before the update, the only Piercing guns available were the Plague/Blitz Needle, and the Callahan. Even though the increased projectile speed makes it much more viable against Fiends than before, now that we also have a Piercing Antigua and Piercing Blaster, there's basically no reason to use the Polaris *or* Wildfire in that scenario anyways.
I can't really advise if it's best to balance this by adjusting status chances or power or what (nor can I tell if such a balance has already been implemented).
I was initially concerned that the Permafroster's Freeze wouldn't be too useful, since the Freeze might interfere with knockback like on the Polaris, and that unlike the Polaris, the status effect is completely removed on hit. However, I found that it was actually fairly effective in-game due to when the third shot was the freezing shot. I could keep enemies at bay, as well as plan or respond positively if the enemy was frozen. It actually worked out fairly well.
I have concern for the set as a whole, however. When I first tried out the Polaris in regular SK, I was completely astonished. It had huge knockback, a 3-shot clip, high damage, status effects, was quick to shoot, and allowed the user to move while shooting. There were very few drawbacks. The Shock did require users to have to adjust their distance if it cancelled knockback, the slow projectile speed made it difficult to use on Fiends, enemies like Scuttlebots would run into the player's space making only the close-up shots effective, and the knockback made it difficult to use in a party. As a Polaris user, I do believe it deserves a nerf (I think clip would work), since the existing drawbacks aren't effective enough. It promotes a "fire and forget, spam away" mentality that has earned the Polaris a real bad reputation, and nothing in this update addresses that.