Hi there!
I'm not sure if all the threads from the old forums made it into the new "Nation" setup, but if you do a search for P90X you should turn up a LOT of threads that describe it in detail. Also, there are some coaches on this board who will very likely chime in next time they visit.
I have P90X, and here's my take on the system. This is just my opinion.
The package includes 12 DVDs, a nutrition plan, a workbook and a calendar for tracking your workouts and recording your progress.
To start the system, I needed to buy a pull-up bar (I bought the door gym from Amazon, about $40, but there are lots of choices). I already had a variety of dumbbells and a barbell.
The nutrition plan is very much like "The Eat Clean Diet" or any lean protein/veggie/complex carb plan of that ilk. So, probably effective and helpful if you need guidance in that area.
P90X works using "Muscle Confusion", which means that you do essentially the same workouts for three weeks, take a rest week, then start a different cycle for the next three weeks, then another rest, etc. You master one series of exercises, then present your body with a new set of challenges. The theory is sound, as far as I know. It's similar to the principle behind STS and the mesocycles.
Each workout is about an hour in length, including a warm-up and cool-down stretch. Ab Ripper X is only 15 minutes, and you are encouraged to use it as an add-on.
I was already in good shape when I started, and I didn't notice any HUGE improvements, but I also didn't actually do the whole thing as written. One change worth noting: at the beginning, I could do NO pullups. I can now do three to five, depending on the day.
I admit that I have a bit of a limited attention span and I get bored pretty easily. I rarely stick to a rotation for more than a month. Six weeks is pushing it. 12 weeks...well, it was never going to happen. I tried all of the workouts. Some were pretty challenging and fun. Some of them I just didn't like. At all.
I liked Tony just fine as an instructor, and all the workouts were sorta challenging, but personally (I emphasize "personally", because I know many people on these forums REALLY LIKE P90X, and it really delivered results, and each person's experience is unique- again, just my opinion), I didn't find any of it as challenging as Cathe's workouts. You are encouraged to select challenging weights and to push yourself, and I'm totally into that, but even going full-force, crazy-hard into some of these, I felt like they were just slightly easier. Still effective! But easier.
Workouts I did once and will never do again: P90X Ab Ripper X, Back & Biceps, Cardio X, X Stretch, Yoga X. Ugh.
Two that were just OK: Chest & Back, and Chest Shoulders & Triceps.
Ones I liked: Legs & Back, Shoulders & Arms, Kenpo X, Plyometrics and Core Synergistics. Especially the plyometrics and the core synergistics. I include these in rotations all the time.
So now you have a detailed opinion from someone who loves to workout hard, is in good shape, and prefers Cathe to all other instructors. Who has a sufficient amount of focus to complete a one-month rotation, and doesn't really care for pull-ups and push-ups. I like the low-tech, traditional gym-style approach, but I guess I didn't appreciate having 90 days of workouts and nutrition planned out for me as much as I thought I would. I like a bit more flexibility and surprise in my program. I'm more excited about STS, because Cathe only schedules three of my weekly workouts and I can do as I please the rest of the time. I think I'd be more likley to adhere to that style of program.
It's still a solid bunch of workouts that you can use to change things up, and heck, maybe you would like some of the stuff I didn't like? Hope this helps, and good luck! Let us know what you decide and what you think!
Lisa