I haven't worn them, but know many Marines and Sailors who do. I get beat on a weekly basis in 10k's by a Coastie who wears them (last week was 36 mins for him, 45 for me).
From the people I know who wear them:
They take a while to get used to. You aren't going to go out and run 5 miles the first week. Your feet WILL fatigue faster at first. As you learn to run in them, your feet will adapt. I know one guy who does half-marathons in them, but not full marathons. Says he can't get his feet to stay healthy in them past about 15 miles.
They make you run on your toes more, like you're supposed to. It takes different muscles to run in them than with regular shoes. Supposed to be less problems with stress fractures, but who knows...
They are the "updated" way to barefoot-run. I've never tried it except on a treadmill and my feet/legs were DEAD after 1/2 mile. I need good running shoes with arch support or it kills my shins (not shin-splints...stress fractures in the bones)...hence I haven't tried the 5 Fingers. Maybe they'd make me run different and it'd be ok...can't really risk it though as if I go med-down, who knows what they'll do to me.