Anyone can shed weight with proper nutrition and a kick #$% training program. The nutrition part of it is BY FAR the most important though. You can actually lose weight just by changing your diet. The P90X workout is certainly not a bad program, but I always advocate picking up and moving heavy things in the weight room. Nothing beats it for increasing testosterone levels, bone density, and lean muscle mass. You also get the benefit of a higher EPOC(excess post-workout oxygen consumption), which is basically your body metabolizing fat and rebuilding your muscles. The workouts themselves are also a bit to long in my opinion. Your muscles generally run out of ATP(energy) after about 30-45 minutes of high intensity exercise. That is why I am a big advocate of "less is more" when it comes to training.
All that being said, I still believe that P90X is a great way to get started if you haven't worked out much. I would never tell someone not to do it if they enjoyed it. It's certainly more than 80% of Americans are doing.