After running exclusively in VFFs for 6-months, I've thrown them in the garbage and gone back to flats. Never been happier.
I guess I was curious to see what all the hoopla was about and they were pretty funky and I did like the feeling of the road on my feet, even when not running. I built up to ~50 miles per week, all road and at the same training pace I was doing previously.
I never got injured, but I have to admit, my feet felt "creaky" in the morning and it took 1-2 hours of moving around until I got a spring in my step. The biggest impact of this is that I didn't want to run at 5am, whereas previously I would roll out of bed, put on flats/trainers and hit the road.
I didn't have to adjust my mechanics, because I've always been a mid foot striker and I've always liked a low delta between forefoot and heel.
I decided to quit and move back to trainers for a multitude of reasons. I felt like they were hindering my training (like to run at 5am), I was concerned about increasing my mileage beyond 50 mpw, I felt slower in them (more effort to maintain my training paces), I don't see any of my peers wearing them (at the front of races), felt like a "follower". Most of the folks I spoke to online about them were much older, heavier and slower. I felt like it was a religion (BF/VFF).
I have to laugh at myself and ask "What was I thinking?", but I am glad I tried them and can speak from experience now.