I'm currently doing the p90x "lean" program and am finishing up on my 3rd week. So far, its fantastic! I have nothing negative to say about it at all.
It IS hard work and you'll find yourself drenched in sweat after almost every workout. My favorite workouts are the Core Synergistics and Ab Ripper X (so far). Least fav so far is yoga...mainly because I am not flexible and it is an hour and a half long (but at the same time it teaches you patience and poise).
The program basically has you do a pattern of calisthenics one day + abs, then cardio the next day, then calisthenics +abs, then cardio.... etc. I think it helps decrease the chances of burn out or boredom.
If you do plan on doing it, don't be afraid to cross-train!...do something else once a week (aka, hiking, jogging, biking, etc). I do this because I have ADD when it comes to working out, so the more variations in my workout, the better.
Good luck if you do end up trying out the program! I'd say to definitely go for it.