My girlfriend had some significant issues with an attempted transition towards Nike Frees. The higher impact on the foot gave her sesamoiditis and a stress fracture near the big toe. If you have a foot structure that's not the norm, it's worth thinking twice about these.

Rotating your shoes is one of the best ways to prevent injuries if you're trying to move towards more lightweight shoes. I try to use a combination of racing flats, Nike Free 3.0s, and trail flats, and variety really does help cut overuse injuries. If you intend to run fast on trails, you can get many of the benefits(e.g. low weight, flexibility, and a more responsive foot feel) with a trail flat like the inov-8 X-Talon 212, while avoiding a lot of the catastrophic injury possibilities.

One final note: although I love my pair of Five Fingers KSO, they smell really bad very quickly. If you get a slightly larger size, some people have luck using the Injinji toe socks with them.