First off, P90X is definitely an advanced series, NOT for beginners! What I like about it most is that it is a rotation that is already set up, covering 90 days. The workouts are:
weeks 1-3
chest/back (lots of push-ups and pull-ups in push/pull fashion, but modifications shown for pullups using a band) and Ab Ripper x (pilates-like core work and crunch-less ab work that is killer!)
plyometrics (intense cardio/plyo)
shoulder and arms (I LOVE the set up of this workout: go from shoulders to biceps to triceps, repeat, then rest and stretch before oging into a second cycle, 4/5 cycles total)
yoga X (a tough 90 minute yoga routine)
legs and back (lots of unique leg moves, with light weights or no weights, but they are effective!, alternating with back work no exercises are repeated).
Kenpo: martial arts. Not a tough workout on its own, but following the leg day, and with DOMS still hanging on from shoulder and arms, it's effective.
stretch X
Then you have a recovery week which includes yoga, stretching, kenpo, and "core synergistics," a very unique routine with light weights, lots of core moves.
The next cycle (weeks 5, 6, 7) substitutes different upper body weight workouts (chest/shoulder/triceps in a cycle, and back/biceps, no exercises repeated), but is otherwise like weeks 1-3.
I'm in week 7 of the rotation, and my abs have never been in such great shape, I've got more muslce definition, I'm more flexible, and my geetchy shoulder has taken very well to the stretches and shoulder exercises.
Granted, some of these results could be attained, maybe, by a differnt rotation, but the P90X one is so well set up. I'm even going to use it as a template for future rotations (ie: Cathe's Slow and Heavy could sub for week 1-3 weight work, and PS could be used for week 5-7 weights, though the cycle set up of P90X is so much more interesting that doing one exercise for three sets, then going on to the same body part, and finishing it off before going on to body part 2).