KSO - or "Keep Stuff Out" model. It just seemed the most versatile at the time. And I did quite a few different things in them - from some light running, to wearing them to work, to taking them along on a backpacking trip as "camp shoes."
As far as the running goes, I didn't put a ton of miles on them - I used them once a week or so, for runs up to about 3 miles, mostly on pavement. It's only recently that I've worked up to more than that, and off-road use as well.
They've held up well - I did have a seam blow out on one of them, but some quick work with a needle and thread took care of that. One funny thing about them is that they tend to develop an odor after a while - I don't have smelly feet in general, but these tend to soak it up. They are machine washable (I throw them in with some towels) and that tends to freshen them up a bit.
The next pair that I bought was just recently, at an REI garage sale where they off-load returned and demo equipment. They were a pair of lightly used TrekSports, in one size smaller than the KSOs. (Because of my Morton's Toe, I'm in between a 46 and a 47. The longer second toe makes the 46s a little snug on that toe - I can't wear the TrekSports with socks - but the 47s can feel a little floppy sometimes.)
There's not a huge difference between the uppers between the two. They have a similar strap pattern, and similar insole. The TrekSports have a couple of refinements over the KSOs, though. First, the tops of the toes are reinforced with a shiny material. I'm not exactly sure what that's supposed to do, since I haven't had a problem with that area. Second, there's a heel pad on the TrekSports. It's perfectly comfortable - but again, I hadn't seen a problem with that area on the older model.
The main difference between the KSOs and the Treksports is, of course, the soles. Here are the graphics from Vibram's website for each model:
For me, the TrekSports are the best all-around answer in a FiveFinger model. I would love to try some of the road-specific models, like the Bikila, but I would need to be much more committed to the minimalist thing before I buy another pair of these...