The KSO (Keeps Stuff Out) is the original VFF. A 2mm insole plus 3.5mm of Vibram rubber provides the closest thing to barefoot running that you can get. All the individual bones and muscles in the foot function without restriction. They look a little odd, especially with the separate toes but you get used to them. A word of warning, going from a pair of thick insole, cushioned high mileage classic running shoes to these is a shock to your feet and calves. You should start with JUST 10 minutes at a time. Nobody wants to run for 10 minutes so I used to carry my normal shoes and then change after my warm up, then carry the VFFs in a bumbag. There is lots of info on the transition away from over cushioned shoes out there, check out google and youtube. You should consider using a pair of inov-8 shoes as a intermediate stage, choose a pair from Shoc-Zone 2 or 3 and also transition into these in order to avoid a lower leg injury. I use my KSOs as kind of a workout for my feet and calves, sort of a running strength session. Having run off road in them once on wet mud I never will again, that’s why we also have the Treksport.
The Treksport can be looked upon as the off road KSO. They have a 4mm vibram sole and a 4mm midsole, which is twice the cushioning of the KSO (although that is twice not very much at all). The sock is also a thicker, more durable construction with a higher performing fabric being used. In concept and use the Treksport is the same as the KSO, just for those who don’t want natural running to be confined to artifical surfaces and decent paths.
I have found that my feet and ankles are stronger after a year or so of using minimalist footwear, this has been combined with regular yoga which I am sure has also contributed. However the single biggest benefit from running in VFFs for me was learning how I landed on my feet and how that varied with gradient, terrain and fatigue. If you are badly heel striking in a pair of VFFs then you really know about it but you also learn about all the other places you land on your feet and how to change your stride and speed to get back to landing nearer your fore foot.