I read Born to Run, and the basic argument made all sorts of inherent sense to me so I got a pair of Five Fingers about four months ago.
I'm frankly too big to run a lot (190 pounds of lean, rippling muscle...) but I do it anyway to keep myself in check, mostly running home after work during the week. So, that's about seven miles on New York City's finest concrete four times a week, and I do all that running in big, fluffy, Asics Kayanos.
I take the Five Fingers out once a week and do a 3.5 mile loop of a local park and try to stay on dirt pathways.
I cannot tell you guys how weird it was to run in the FF the first time - the analogy of learning how to run all over again is 100% apt. I found myself running almost entirely on the balls of my feet, while my stride got shorter and quicker.
You can literally feel the difference it makes to move your head forward and backward, to move your back forward and backward, to rotate your hips forward and back, to move your shoulders... It really is remarkable.
In a clear indication that it uses entirely different muscles and posture, I was in a huge amount of pain after running in these shoes and for the first two months it took a full two days to recover.
In particular, the muscles in my calves were in knots and the tendons in my ankles were killing me.
Now, the pain was the good pain (and not "injury pain") - the pain you feel two days after slacking for months and then hitting the gym and doing some deadlifts. Still, I had to walk down stairs backward.
The best these shoes have felt is a couple of six mile runs I did, just a couple days apart, on the grassy / mossy roadside of Fundy National Park while camping.
I think the softer ground spared me a lot of impact, and my body really got into the rhythm of running.
I don't know that they are a panacea - I suspect I would likely do a tremendous amount of damage to my body if I was to run in the city or on any kind of paved / hard surface with these shoes.
But for people who have access to softer surfaces to run on (or who weigh a lot less than me) and enjoy running, I really have to suggest trying them.
Try them, however, without the expectation that these shoes will cure all your back/knee/joint/kidney/hairloss problems... rather, expect that they are a very useful tool to teach you about how your run and a means to explore how you can run differently.