I've done 3 rounds of P90X (with some program modifications of the last two rounds) and I always get leaner every time.  Definitely a good, solid strength training program.  Some workouts are nice, classic resistance training and others are good for focusing on your weak spots (like yoga).  I used to hate the yoga, but now I love it.

The P90x program is a classic example of overtraining.  You are putting your body under such heavy stress for 5 days a week, then stretch it out on Saturdays. 

Also, when you stop doing P90x, your body is now accustomed to the workouts and that level of intensity.  People who stop the program and return to "normal" activity, tend to put the weight back on at a faster pace.

I have seen this first hand with people I work with.  They looked great when they were on the program, then about a month after they stopped, they put the weight right back on.

It really all comes down to your diet, 15 minutes of intense exercising 3x a week, and watching your cortisol levels.