I use these in my Vibram Five Fingers mostly to keep them from smelling as fast.  They work well for that, especially when it is hot out.

I don't like how I can't feel the ground nearly as well with them on, although I am getting more used to it.  I have three pairs that I rotate between, I have worn them each about 5 times, and one pair is already developing a hole in between the toes.  They will end up being very expensive socks if they don't last very long.

Pros:
-Keeps Five Fingers from smelling as fast
-Keep feet cooler in hot weather
-Help prevent blisters

Cons:
-Can't feel the ground as well
-One of three pairs has begun to develop a hole after a few uses
-Injinji logo is clearly visible above top of shoes