I've had 2 experiences with Pilates:
A while ago, a friend of mine was teaching it (also called Mat Class/Works), and I did it for several months and thought it was a GREAT workout. It is definitely more challenging than it looks, and I really liked it, but it got kind of expensive. I felt more toned, had better posture, and it also helps with breathing (which really is the basis of Pilates).
More recently, I bought a set of tapes (sorry, can't remember what it's called at the moment!) that has an AM workout and a PM workout. I really like that a lot too, but I haven't been consistent with it.
I think Pilates is great because you can do it at home and even a 25 minute workout (like my tapes) is very challenging. My personal opinion is that if you're interested in Pilates that you might want to take a few classes with an in-person instructor before purchasing tapes (I know a lot of fitness centers and the Y have them these days). I know when I was taking the classes from my friend, she really helped me learn how to improve my form and do the exercises the proper way to provide the most benefit to my body, I'm not sure if I would really have learned properly had I just gone straight to the tapes.