I've used both P90X and Insanity.
I did P90X in two different occasions......results were great even though I didn't fully commit to do the routines.
I TRIED to do Insanity and unless you're getting paid to look like the people in the commercials or your life depends on it.....I don't recommended it, is just too brutal! and soon you will hate it and wont touch it again.
P90X is also hard but more manageable.
Also let me tell you that diet is VERY, VERY important; while doing P90X the first time I didn't diet at all and saw results, but the next time I did it along with diet the results were better and faster.
So yeah P90X is fun, hard, works and is different from your average workout.....which reminds me that I need to get on it again as I've gained 35 lbs since getting fired!