A buddy recently finished P90X and lost between 20-30 pounds with it...and gained tons of muscle. Says that the workout is no joke and really works. After he finished the entire deal, he switched to running and weights for about 6 weeks and then went back to P90X to start over.
Don't forget to do the yoga. My wife borrowed it from them and I got tired just watching so thank goodness the beer fridge was close.