I LOVE P90X (and Tony Horton).
I'm on week four of P90X, and while it definitely makes me sweat, the first month wasn't all that bad. Then again, I played competitive soccer from the time I was 7 up through high school, and then was active in intramural soccer, women's rugby, and women's boxing at U of M (GO BLUE), and I just graduated in May, so I had a good base to start with. After the first week of doing the "classic" version, I started running for about 30 minutes on the weight training days, and I'll be switching to doubles (cardio in the am, weight training in the pm) during Phase 2.
I weighed in at 119 (5'6") at the beginning of the program, and now I'm up to 121, mostly because of my increase in muscle mass/protein intake. I don't do P90X for the weight loss (and as a general rule, I rarely get on the scale as I care more about how my pants fit than the numbers), but I wanted more definition, and the X has definitely helped in that regard.
P.S. This is actually my first post on Letsgowings. Hardcore Wings fan and a bit of a fitness freak, too.