P90X is a wonderful program: very balanced (with strength work, cardio, yoga/stretching and core workouts). It can teach you how to set up a good rotation, the importance of rest periods, the benefits of cycling exercises to not just do straight sets for the same muscle group before moving on to the next, and the benefits of recovery weeks.
So far, I'd say it's the best weight training system out there (I'm hoping Cathe's STS will vie for that title).
A few comments:
1) When I first viewed the DVD's, I was very intimidated by the first workout in the rotation---chest and back--because it's primarily push-ups and pull-ups, so I left P90X on the shelf for a while. Then I decided to just try it, but I started mid-week-1 with the shoulders and arms workout. It won me over (and is one of my favorite P90X workouts). (I just caught up with the workouts I'd missed by cycling through all the workouts 3 times in order).
2) personally, I needed more glute work to keep things where I want them to be

, so I added some heavy squats (with my Rockit machine) and/or sprints on a rebounder in the place of back work in some of the Legs + Back workouts. From reading others' posts about P90X, opinion seems about equally divided as to whether there is enough lower body work (in the legs workout, it's mostly unweighted, but legs are also hit in yoga and plyo) or not.
3) The cardio is less intense than Cathe's, but that's for a purpose : too much cardio burns the calories your body needs to make muscle. The "doubles" rotation adds more cardio (though not right away: don't be tempted to do so either, until your body adapts to P90X).
4) there are three rotations suggested: classic, doubles and lean. The classic and doubles are well-balanced, but the lean seems like an afterthought, and each week, some body part is given short shrift (and important ones, like back). If you are tempted to do the 'lean' rotation, instead just do 1/2 of each of the weight workouts (each one repeats exercise cycles, so you'll get all the moves by doing only 1/2) and you won't miss body parts. Some of us call this a "halfsies" rotation.
5) Some people find Tony's warm-ups for the resistance work boring, but I found them to be very useful. They even helped me loosen up a problematic shoulder.