I have a pair. They are NOT for climbing. You could
climb in them... just as you could
climb without any footwear on whatsoever. I would never climb in fivefingers.
Personally, I think they're best suited for the trails. I've done 20+ miles in a day while wearing these, and I thought they were great. I felt a bit more "at one" with nature when I hiked in these. They provide minimal protection for your feet, so you have to be mindful of where you step. They force you to be a smarter hiker/walker/runner.
They can be uncomfortable at first --you'll likely get blisters-- but they're great once you get used to them. You'll learn how to utilize your toes more, and you'll also develop stronger/tougher feet.
These are supposed to emulate being barefoot, so I suggest you read up on barefoot running before investing in a pair of fivefingers. The wikipedia is one place to start: http://en.wikipedia.org/wiki/Barefoot_running