Followed it to the letter, meal plan and all. Like I said, it got me lean and fit, but not muscular. When you are doing way to many sets, or sets to failure, you fry your central nervous system, and your form goes to hell. Ideally you want to lift heavy, but reduce your reps to about 6, or leave about 2 more reps in the tank. Any more than that, and you begin to reduce your gain. You also don't need to do a 1000 sit ups to get abs, in fact, you don't need to do ab work at all if you are doing the correct workouts. Dead lifts and squats use your core for the lifts. Arnold Schwarzenegger and other lifters never did crunches, their staple of lifts was what I just mentioned and lifting heavy.
This is of course dependent on your goals you want to reach, if you want to be lean and fit, then P90X works very well, but if you want muscle and at the same time be fit, then follow the website I posted, you will see that all you need is about 45 min of working out with about 20 minutes of cardio, every other day and you will retain the same level of fitness as P90X. At 39, this is the first time I have made incredible muscle gains while at the same time keeping my fitness levels up.
2012 Focus Titanium Black Manual