If you haven't been training hard or working out regularly for a significant amount of time (anyone without a steady workout routine 3+ times a week for a year or more), you're gonna have a tough time completing P90X.
It is an advanced workout meant for people who need to push the limits, but sadly it is marketed to average joe six pack who hardly gets up off the couch.
It is an amazing program, but I would estimate the failure rate (non-completion of the full 90 day program) to be significant. Probably as high as 90%. Most fall off after week 3 or 4.
I have a significant background in training and in all honesty, I could not even complete it at the time I tried it. I will admit that at the time, I had been out of the gym for over 9 months, due to work related issues. I may try it again this spring, after I finish my current training phase, and see how far I get.
Bottom line, it is not for the faint of heart.