Still not a fan of P90x and as mentioned, it's because it is essentially cardio or at least "too cardio" for my purposes. It's great that it motivates people to start working out and there are several pluses that go along with someone starting P90x:
1. The nature of cardio is that long cardio sessions can burn pretty large amount of calories while typical resistance training has a negligible calorie burning effect. I'd imagine a P90x burns off a whole twinkie or so.
2. Working out and particularly cardio has an appetite suppressing effect for a number of reasons. You're not hungry while you're working out. There is a slight stomach shrinking that goes on during exercise as well. Psychologically, folks feel much more accountable for their diet if they have worked out that day as well.
3. You're working out! Hooray. Untrained individuals in particular will respond very well to just about any workout program that they follow closely, especially if there is any resistance aspect to it.
I still am a much bigger fan of true resistance training, like lifting heavy weights. Improving maximal strength is very important, makes notable changes to physique, and even plays a role in hormone optimization (cardio has been shown to have a negative effect of testosterone, in contrast).
For those worried about getting "too big," please return to this planet (or remove your skinny jeans). Gaining muscle mass is very difficult even with a perfect program and diet. There is a distinct evolutionary advantage to gaining fat instead of muscle and you will fight that constantly. For a trained lifter (2+ years training), gaining 2 pounds muscle mass per YEAR is considered a reasonable goal. If you have gained or are gaining more you are either: untrained, gaining more fat than you think, or you have amazing genetics.
And while it's fabulous to talk about workout programs, it all comes down to diet. Your energy balance (calories in vs out) along with adequate protein intake is going to explain 75% of your visible results. You may gain some strength or endurance with a poor diet, but your physique will change as your diet dictates it to. If anyone is interested in really cracking down on your diet, I'd be happy to explain more. It is actually much simpler than you might think and you'll be happy (or perhaps sorry) to know that much of the popular discourse about diet is completely false or misleading.
