I have many colleagues who have done the program, and the results are excellent. There are 2 main roadblocks to any exercise program:
- Diet
- Motivation
The diet for the P90X program is geared towards progression in the program...so as others have said, change the diet and YMMV as an uncontrolled variable.
More importantly, motivation to continue in any program is one of the hardest things to maintain, and that is one of the strengths of the P90X program - variety and the FEELING of results. By doing the different components, it keeps things interesting, and helps work weak points in such a fashion that you start feeling results all round, and that should fuel the desire to continue.
If somebody does 1-2 hrs of intense workout everyday with a careful diet, it doesn't matter what the program is, they will get some form of results. With P90x, you're working all muscle groups as well as endurance, flexibility, balance, agility, etc so you keep motivated to stick with it.