I just finished 180 days of P90X -- did the "lean" program, took two weeks off, then started on the "classic" program. After four weeks on the classic, I felt that it was a little too easy, so I switched to the "doubles" program week 5 (classic and doubles are identical the first four weeks). Needless to say, P90X doubles is a @$$-kicker -- proud to say I finished it, but REALLY glad to be done. FWIW, I'm 46 and haven't been in this kind of shape since my early 20's.
As far as results, this time a year ago I was in terrible shape -- at least 50 lbs overweight and probably couldn't run 1/2 a mile. I knew I wasn't in any condition to start P90X, so I started Power-90 instead. After that I rolled into the Power-90 Master Series, then did P90X Lean, followed by P90X Doubles. So, a year later I'm 65lbs lighter, running 5+ miles 3x a week, and starting to train for a 1/2 marathon.
So, IMHO, yes, P90X works, but only if you do it. Also, be realistic -- if you're really out of shape (like I was), you may want to start with something a bit easier.
Good luck,
Z