I LOVE P90X! I do all my training at home, and P90X is an excellent program for upper body especially, but also for core (some very innovative core workouts). The 90-day rotation also is very well thought out, and includes strength work (well-sequenced exercises done at your own pace, lots of pull-ups/chin-ups and push-ups for back and chest, effective rest breaks with dynamic stretches), core work ( "Core synergistics" and "Ab Ripper X"), moderate cardio (kickboxing), plyometrcs, yoga (a tough, 90-minute session, once a week).
It all works together very well. I'm on a couple of video fitness forums, and the majority of people who have tried P90X got results similar to me (upper body strength and definition gains, stronger and more defined core). Some of us, me included, didn't find the lower body work enough for us (but the "we" I'm talking about is women, and "we" may need more work in that area to keep the shapes we want...I lost some of the juice in my caboose from doing exclusively P90X lower body, and needed to add in some heavier lower body work. Maybe not a problem for men.)
I never had Power 90, but from what I've read, it doesn't appeal to me. It sounds very basic compared to P90X, which is advanced.
P0wer half hour is another program from BeachBody that is a step up from Power 90. 5 (or 6?) 30-minute workouts that are very good.
P90 Masters is the newest offering in the P90series. It's supposed to be a step between Power 90 and P90X, but the workouts that have come out so far are varied. I really like "Sculpt 5-6" from the previews. Similar weight work to P90X, but in a one-hour,total body workout.
You can read user reviews of these and other workouts at www.videofitness.com
(click the "video reviews" on the left).