Are you trying to gain weight or lose weight? you need to follow very different diet regimens depending on your goal. Both ways you want to be getting a lot of protein and a fair amount of simple carbohydrates (Oats and vegetables) but if you're trying to put on weight you have to do things a bit differently like making protein shakes with milk and peanut butter where as if you're looking to lose weight you'd want to do the same with just water. From what I've heard about the p90x system it will work because it's a workout plan essentially, it takes out the need for guess work. ("Do I need to do exercise this muscle or that?") one thing I've seen a lot of guys do is completely neglect their legs this is a HUGE mistake. The leg muscles are the biggest muscles in your body, increasing their strength is the best way to increase your metabolism. and Increase in muscle is the only real way to permanently increase your metabolism. Don't get too concerned about the "6 meals a day" advice you hear everywhere it's scientifically bunk (your body doesn't change it metabolism that quickly) however if you are someone with a strong appetite that it will help with that. Basically do which ever plan best keeps your portion control where it should be keep track of calories per day. If you're looking to cut you're probably going to want something like 1,800 if you're looking to bulk I've seen people do up to 3,500-4,000 depending.
I did it for quite a while. Never stuck to it though, I lack the resources to follow it through to the end. (I can't follow the diet plan properly.) I really enjoyed the work program as a whole. It feels so good when you've completed an entire session.I can tell that even without following the proper diet plan and just eating good hearty meals and plenty of protein, I was gaining weight and putting on muscle. After the first month I gained about 6-7 pounds. ^^ Hmm, this is making me want to do a workout right now!