Vibram has several styles. For trail running, the KSO works best. My size 43 weight 13.8 oz for the pair. With the KSO, Vibram recommends using the Injinti toe socks, but I find them not as comfortable, as sans socks.
The KSOs do a good job of "Keeping Stuff Out" which is what KSO stands for.
I really like them. They do need to be washed periodically to get rid of the smell.
For hiking in the KSO, I have a tendency to hit my little toes on rocks which is painful. Not a good shoe for rocky trails. Just imagine walking barefoot with all your toes spread out on the ground.
The theory behind the fivefingers is you will run naturally. With conventional running shoes, runners usually run with a heel strike, which leads to injuries. The fivefingers do not force you to run with a ball strike, but the bone jarring impact from a heel strike forces the body to subconsciously run on the balls of your feet, which allows the legs to act as a spring. This change in running will soon cause calf and other muscle soreness, until you build up those muscles. Also if you are not used to walking barefoot on in flip flops full time, it takes some time to build strength in the arch.
For all around trail wear, these are a one-shoe-does-everything solution, as long as the trail is well maintained and you have worn them daily. Do not undertake a long hike without wearing them everyday for a period of time.
I have switched to cross country running flats for most backpacking, because they are lighter than the fivefingers, have more support, and shoe laces provide more fit adjustment on a trip. I recently did a tough 60 mile hike in racing flats. I would have had foot problems, because flats have little support. But no problems encountered since I had been wearing the fivefingers daily for footwear and running workouts. The fivefingers had built up strength in all the components of my feet, which allowed me to hike in flats.