I've read blog and forum posts emphasizing the importance of following P90X to the T, and never deviating from the program (other than by switching to the "Lean" or "Doubles" routines if desired). The materials even warn that users shouldn't linger more than one extra week on any set of weekly workouts, suggesting that your muscle development will "plateau" if you stick with one set of exercises too long.
While I don't disagree generally with the idea that it's useful to introduce variation in workouts, my sense is that: (1) no one's in danger of "plateauing" after a mere three to four weeks' worth of performing wildly different resistance and cardio moves, (2) "muscle confusion" is -- at least in the P90X context -- a marketing gimmick that should be more accurately termed "variation for the sake of staving off boredom," and (3) in any event, the P90X schedule seems to run counter to the tenets of "muscle confusion" given that the only resistance programs that change involve the chest, back and arms.
But we've seen the awesome before-and-after photos, so without "muscle confusion" driving results, what's going on? My theory: You can take just about anyone and put him/her on a strict diet and heavy-duty daily exercise regimen (8 hours of hardcore exercise per week qualifies as heavy-duty in my book) and you're going to see body changes. Fat will be lost. Strength will be gained. Fitness will improve. But my guess is that slavish devotion to the P90X plan isn't necessary.
That being said, I'm going to continue following the P90X plan as faithfully as possible. Right now, I need the motivation and the structure provided by Tony "King Douche" Horton to keep going, and it's a hell of a lot easier popping in a DVD every morning than it is to go to the gym and figure out how to create a workout that equals P90X in intensity and variety.