Yes I have tried it. I started to work out about 3 days a week when I was in college. I kept with that for about a year after graduating and then decided to cancel my gym membership to save some cash and try out the DVDs. One of my roommates has them and I borrowed them.
I got through about 9 weeks of the 12 week program. It's alot of work and it's tough to stick with. For someone who only works out casually before trying out this program, that first week you're going to be VERY sore when you wake up each morning.
I know for me, I found myself skipping a day here or there, skipping an exercise or two, but thats ok. Don't overdo it, work to your personal pace. I imagine it to be extremely hard to actually keep with the official p90x schedule.
After 9 weeks, I definately feel healthier, definately more definition in my arms and my chest. However, I'm not one of those guys from the info-mercials that automatically looks massive with big arms and a 6 pack after completing this. Alot depends on genetics, body type, and diet. Emphasis on diet, don't think you can eat fried foods every night, do this during the day and think the program will be a success.