Whatever diet/exercise program that actually works for somebody is good, by definition. That being said, I generally am opposed to diet/exercise programs that require you to spend a lot of money. I’m not familiar with the direct costs associated with P90X, but I have heard that the diet prescribed in the program is expensive, which a healthy diet does not have to be.
Also, the program’s trademark “confusion” system of surprising your muscles is a good idea, but you can get the same results simply by varying the number of reps you do and the amounts you lift.
As for diet- I try to eat healthy, and I keep my weight and my stomach under control, but I’ve never used a diet program. I don’t knock them on principle- spending the money, having the guide, or even just being part of something bigger can be good incentives for people. But they aren’t necessary to lose weight or keep in good shape, and so I think most people should try to do it on their own first, and save the money. The keys are portion control, eliminating certain problem categories (trans fats, MSG, and high fructose corn syrup are all really bad for you, and I’ve found that after cutting them out of my diet, I don’t even crave them anymore, and I don’t feel good after eating them), and letting yourself indulge once in awhile. If you are craving pizza, eat pizza. Have a slice or two. You can try to satisfy that craving with something healthier, or get healthier toppings on your pizza, or what have you, but if you want one particular thing, rather than make it a giant THING, you should indulge that craving, moderately, and once in awhile. I’ve found I have much more success that way.
OK, off the soapbox now.