I started p90x last summer but, when I got busy, I stopped. The next time I do it I'm going to change my approach.
First, I already do kickboxing, spinning, running, some swimming so I'm not going to do any of the p90x cardio this time around. I might do plyo but kickboxing does some of that.
So, I guess I recommend making room for p90x in your current routine rather than adding an additional 7 or so hours of working out to a week where you might already have some hours logged. Focus on finding a sustainable pace.
Also, I wouldn't pause as many times as Tony Horton tells you you can take a break. The first time, I would do the entire video and max out reps and it stretched the workout to twice as long sometimes which becomes really inconvenient and something I'm way less likely to follow through with. Do what you can in the hour the DVD plays for and work your way up.
They also have you doing ab ripper at the end of the resistance workouts. What worked better for me, since resistance training is the thing that's missing from my workout, it is the hardest part, anyway, what worked better for me was stopping the DVD, and my player remembers where I stopped, and coming back the next day and doing ab ripper when I got home after cardio.
It really works though. I felt much stronger and could see definition after the first month, and, like, the middle ab ridge. Also, I had to keep tightening my belt and, I do yoga already, but I could feel better posture as my core and back got stronger.
Find a way to stick with it and have a plan for what you're going to do afterwards, whether thats another round of p90x or something else. The changes you are looking for will be beneficial for all your life, not just for the next 90 days.
And if you eat well already, you might read what they recommend and keep it in mind but, if you're anything like me, gradual changes will last longer than adding all this stuff at once.