Vibram recommends the KSO Trek for "light trekking, trail running, fitness walking and travel." I have used the shoe for three of those four categories (I'm not sure what fitness walking is) and it works great for all of them.
The sole is lugged 4mm Vibram rubber which is both tough and very flexible. The upper is suede kangaroo leather. There's no lining, so the leather is against your foot. There is a single velcro closure across the top of the foot.
There is another model which Vibram still makes called the TrekSport, which uses the same sole with a thin mesh upper similar to the classic KSO. I don't like it as much.
The fit is pretty consistent with other FiveFinger models, running maybe a fraction of a size larger than the classic KSO. I am a 42 in both but my classic KSOs are just a little bit more snug than my KSO Treks.
Overall, in my opinion these are by far the most comfortable model of FiveFingers. The kangaroo leather feels great against the foot. They also breathe well; I don't have a problem with my feet getting too sweaty. My only issue is that since the shoe isn't lined I have had some issues with blisters on longer runs, but I've never had a problem except when I was putting in a lot of mileage in them.
Much more than any FiveFingers model, the KSO Trek is built to last. I have had my pair for almost three years now and they aren't showing any wear yet. This is really what you would expect given that almost the entire shoe is made out of leather and Vibram rubber. Getting the leather wet doesn't hurt it. In fact, the shoe is machine washable, although I have always just rinsed them off and left them out to dry.
Let's start with running. These shoes really perform well on the trails. The sole is very flexible and lets the foot function as well as any shoe I have worn. Groundfeel is excellent, much better than something like the Merrell Trail Glove, but at the same time the sole provides good rock protection. Traction is also excellent, due both to lugged sole and great proprioception (feedback) you get from the groundfeel and flexibility. The only reason I don't use my pair more for running is that they tend to give me blisters on longer runs. However, on runs of ten miles or less they are a real joy to run in. The lugs are small enough that they work fine for running on roads as well.
Even though my pair is probably going to last for many years longer, it still broke my heart a little bit when I found out that Vibram was discontinuing the KSO Trek. However, if you act quickly this can be to your benefit since they are currently on clearance at REI and elsewhere. Go get them while you still can.