I have done P90X Three times in two years. It is a great system. In the summer months, I tend to be all lower body (Running, Cycling, and Kung-Fu), but in the Winter it gives me a great opportunity to work the upper body and balance myself out.
However, I just started it back up yesterday, and managed to pinch/tweek my Sciatic Nerve by being stupid and not properly lifting and moving my dumbbells. I am the dumbbell. As soon as I heal, looks like I may just go back to Yoga and Tai Chi before jumping back to weights.
Good form is key!