I did a P90X rotation last year, and will probably do one agin. I didn't get the supplements or other crap, but I did check out the macronutrient (protein/carbs/fat) breakdown of their "recovery drink," and found that a soy meal replacement drink that I use is about the same, so I drank that after all the weight workouts.
I did the "Classic" rotation. The "lean" rotation seemed rather incomplete to me, as each week you skip one body part.
My "90 day" rotation ended up being about 100-110 days instead: I added more rest days, sometimes did a supplementary cardio day. I ended up skipping Ab Ripper about as often as I did it, but I still had great ab results.
I visited the forums a couple of times to ask questions, but it seemed like no one with actual answers (or more than I could figure out myself) answered.
The first two weeks, I REALLY needed more sleep! About an extra hour a day. If you are considering doing the "doubles" rotation with extra cardio, I recommend not doing it until week 3 or so (though I think the extra cardio might not actually kick in until week 2 or 3?). Otherwise, you can overdo.
When I got P90X, I previewed all the workouts, and I was SOOO intimidated by Chest/Back that I put it on my shelf and "forgot" about it for several weeks. Then one day I decided to jump in, but I started with Shoulders and Arms, which looked let intimidating, but fun. That worked well, and I continued the rotation from there.