Yes, P90X absolutely works when done correctly - e.g., you put in 100%
It's intensive and it requires some equipment, though - dumbbells and some other random stuff - and it can be difficult starting out for someone who hasn't worked out in awhile. It can also get expensive; my husband found our set on Craigslist for 1/2 the retail price, and eBay and yardsales might also be good places to look.
My husband is a soldier and his unit regularly uses P90X (and Insanity, which is made by the same company but doesn't require dumbbells/extra equipment) for PT, if that helps