Our local Y just got in some Gravity Machines and will be starting classes in the new year. My PT however has taken me down and run through her program with me a few time to get her use to it. I think the machines are great for upper body - not so great for legs as most of the stuff I have seen done so far has been squats so you don't have to stabilize yourself as much as if you do them free standing. Overall though I think the machines are great. You get resistance by the slant of the machine and believe me - there is a big difference even between a 7 and an 8 when doing biceps.
So if you have access to one of these pieces at the gym - if classes are not offered see if you can have a personal trainer go over some exercises. If anyone has used and has good leg exercises let me know.