I consider myself a bit of an expert on treadmills.

Nuero physically they are not a very good idea to get fit on, what happens is the machine moves you, you do not move the machine. There in lies the problem, the metatarsals dont get the same response as you move running, therefore neither does the knee, long term it will cause injury. These injuries will be worse than the ones that your on the machine to rehab, and the muscles dont work as naturally as when you move yourself.

There are one or two other things but you get, (i hope), the general idea.

Regards