I am on the very last week of the 90-day classic rotation. P90X is a great system and well worth getting. I got it because there were so many rave reviews from a LOT of Catheites, which is a rare thing.
I did not do the diet at all, and I subbed Cathe and various yoga workouts here and there, mostly due to time constraints. With most of the exercises, you choose the rep range and pace, and background exercisers continuously show modifications which can still be quite challenging. I'm an ecto and my main goal with this series was to increase overall strength (so I stayed in the 8-10 rep range) and gain a bit of healthy weight. Before I started the rotation, I was a geek and took before pictures and did the fitness test, and I plan on doing the fitness test and taking pictures again when I'm done this week.
As far as results go, I definitely gained strength and definition, most significantly (and surprisingly) in my core. The core is emphasized to a degree in almost every workout and this emphasis, along with the excellent workout, Ab Ripper X, really kicked my core strength up a notch. The few times I subbed a Cathe ab workout (e.g., PUB), it was much easier.
Like Kathryn said, the cardio was designed to be less strenuous. Plyo X is very good, but I didn't care for Kenpo X, so I subbed KPC or Cardio Kicks. I never tried Cardio X.
Unlike Kathryn, my butt is rounder and higher than ever, even though I pretty much did Legs & Back for my legwork. Two times, I subbed the L&G/KPC premix when I decided to combine leg work and kickboxing due to time constraints.
I got P90X from the Ya Ya Swap, but I've heard the price has come down so it's more affordable. I believe the system belongs in every advanced home exerciser's video library.