I think P90X is a fantastic program. Years ago, before I got into running, I was an exercise video hound - mainly The Firm and other strength+cardio routines. P90X is definitely a step up from those types of workouts, and with it I saw and felt changes in my body - both form and function - that nothing else has provided. I did the "lean" version of the program in combination with running, and I was very pleased with the results.
As others have noted, the core benefits are awesome. Not just the abs, but the increased strength and flexibility in my back is noticeable in my running, skiing and biking. I noticed that overall my posture improved as well.
I didn't lose much weight, but I also didn't follow the nutrition portion either. However, I received so many positive comments - from people I hadn't seen in awhile as well as those who see me every day - that I figured something good was going on. Yes, it required dedication and commitment, but for me, it was so worth it.
This spring, I lent my dvds to a friend and just yesterday I asked for them back. Now that my running 'season' is winding down, I want to refocus on getting that core strength and overall fitness back.
As a final note, another purchase option - with a 30-day money-back guarantee - is QVC, of all places. You'll still pay the $$, but you'll be able to try it out for a month and know you can easily return it if you don't like it. I heard horror stories about those purchasing from Beach Body and having difficulties returning/getting a refund. Here's a link.