Swiftstrike Buckler: Thoughts?
I am one who feels that the best defense is a great offense with my Fallen armor and my Khorovod. Thus, when I seen the Swiftstrike Buckler, I felt that I really needed one. Granted, it won't block a single thing in Tier 3, but if I'm overwhelming just about any enemy with the speed of my strikes, what does it matter?
And so far, I've been really happy with the results. My Khorovod has an Attack Speed Low UV, the Fallen set gives Medium Attack Speed (For both my swords and my gun!), and the Swiftstrike Buckler gives a High attack speed (again, to all weapons). This gives my Khorovod max attack speed while giving "ultra" attack speed to my other weapons.
The only problem with this: Devilites. Since the Fallen set makes me deal drastically less damage to Fiends, and since I can barely block any damage in Tier 3, they are the most problemsome enemy. Which is why I never go alone in Devilite levels. :P
Anyway, I was wanting to know the overall thoughts on this shield? It's probably the one I'll be using for the rest of the game, even though the Barbarous gives way more defense and attack damage. It's hard to beat the High Attack Speed boost.
Well if you still like the Speed increase but want the Barbarous Thorn Shield's stronger defense and attack increase then you can just wear the Vog Cub set which will give your speed a one level increase over solely having a Swiftstrike Buckler.
Thinking about it, the Fallen set increases your speed up to Medium while Vog Cub brings it up to Very High (Ultra with your UV). Of course if you use weapons that aren't swords then the Fallen items help those weapons' speed too unlike the Vog Cub.
Its really a tough decision to make. You can have amazing speed with all weapons while suffering from crap damage against Fiends and not having a potential attack power increase or you can have your swords solely be your fast weapons while having a better shield and a sword attack increase. You may just want to try it out first to see just how terrible fiend battling becomes. If its not all that bad then I'd say you're fine with what you've got, otherwise you should probably give consideration to a Vog Cub set and Barbarous Thorn Shield.

Is the damage penalty on Fallen Armor really that bad, even if you use a weapon from the Flourish or Snarble Barb lines?

If the damage penalty from Fallen Armor is really as bad as it sounds, maybe you should get the Vog Cub set.
I already got a Sealed Sword and the Avenger recipe and because Avenger is so slow, I am actually planning to get a Swiftstrike for Tier 2. So, is the speed boost significant meaning you can really notice it? I just hope they get a 4* and 5* version of it in a future update.
Here's a stupid (I think) question: You can bring a 3* shield into T3 if you have a 4*+ shield? I thought you have to equip the 4*+ shield if you want to go in T3.

You need to meet the requirements to gain access to tier 3, but after that, you can do what you want.
If you're using swords and guns, no bombs, you can get the same attack speed bonus as Fallen gives you with one piece each of Vog Cub and either Nameless or Justifier. As far as I can tell Swiftstrike and Fallen do nothing for bombs, but if you use sword or gun (not both) and bombs, you can easily swap out one piece for either a piece of Volcanic Demo or Mad Bomber, and get medium charge time reduction. If you go swords/bombs, one piece each of vog cub and volcanic demo gives you pretty good fire resist, too.
If you carry swappable gear, I would recommend carrying a Dread Skelly Shield for dealing with devilites.
Yes the penalty is pretty bad against Fiends since each piece's damage reduction (mind you for all weapons) is Medium, adding to a Very High reduction for the set, that paired up with a weak shield like the Swiftstrike means that you'd have to avoid Fiends since you can't just take them out quickly and you can't strike a few times and then shield for too long a time.
Oh dude, I totally use a swiftstrike+fallen combo too. Having IAS:Ultra with all weapons is really fun. My weapon set is [Khorovod CTRMed/Callahan/Valiance], DA, GF, and the triple slash trick (http://forums.spiralknights.com/en/node/13083) is super nice with that kind of attack speed.
For a while, it made me feel like my Barb Thorn was a waste of time/money too, since I could just poke in and out so safely - in fact, I really couldn't play without anything higher than IAS:High for a while because I kept trying to move when I was in pure animation lag.
It's really nice when you're soloing, so long as you choose your gates wisely :)