I have no experience with the P90 series, but I have been doing Shawn T's "I only refer to myself in the 3rd person" Insanity Workout. I liked it enough that I went ahead an picked up the extra 3 DVD's that include the Insane Abs, Upper Body Weight Training, and Sports Fitness Something or Other.
The Sports Fitness I gave to a friend who plays basketball for a D-II college.
The other two I do every other day, with the normal Insanity workout calendar thrown in.
My biggest obstacle is that I have a bad back and I can't do every single workout in the Insanity series. I do however enjoy the hell out of the ones I do.
I'm like the guy above, skinny and wanted to put on mass while dumping body fat. I lost a full belt loop in two weeks, and I haven't quit eating Long John Silver's two fish and 3 chicken deep fried heart attack once a week.
The weight training gives you pretty quick results as well. The problem of course is that you need a full set of dumb bells to do them, and what you buy in your first week will be too light in the second week if you do the workout every other day.
Before I started Insanity, I was doing push ups, crunches, running 3.5 miles a day and some other light workouts. Insanity kicked my *** all over the place and back. You'll know you are getting results when you can keep up with the warm-ups.