I have Power 90. I don't follow the plan though because honestly, I find it boring. I tried. The workouts are good and you'll get results if you do it. I did 2 days/week of the strength and 2 days/week of the cardio and saw a difference fast! The plan calls for 3 days/week of each. Now I do each one once a week as part of my plan. I need more variety than just 2 workouts. I'm easily bored.
As for P90x. I do some of the workouts but not the whole plan. Yes, the power 90 prepared me but P90X is tough no matter how prepared you are