To add my $.02 worth, I'm starting phase 3 of P90X, and can vouch for its effectiveness.
I have only lost about 10 lbs, but I have moved a lot around (and muscle weighs more than fat, so there you go).
I have gone down a pants size, and have added a lot of size to my arms and chest.
I started with the bands, and it was a good workout, but I recommend dumbbells. The difference is noticeable.
Hope that helps.