I know two people who have done P90X and got great results. The first, a friend of mine, does it consistently and she looks amazing (she also eats healthy, stays active, and is somewhat obsessed with being and looking fit). She just started the new P90 workout DVDs that recently came out.
My cousin also tried P90X and lost a LOT of weight BUT he only did it for one cycle and went back to eating garbage. He's gained almost if not all of the weight back. I think the only way these DVDs work is if you're watching what you're eating and keep using them (or, after you're finished with them, find another way to keep active).
I have a set of P90 DVDs but I never stuck to it for longer than a week. It is a good workout and I like that you can modify the routine to suit your capabilities.