We bought this for use with P90X as well. My wife used it successfully going through it once, although I suppose she didn't apply as much pull as I was able to. Within 2 weeks of using it correctly, braced through a door (using a third-party door attachment, which was not the problem), the left handle plastic tube that feeds through the grip (which is hollow, and the nylon fabric passes through it) cracked. I was by no means pulling it so hard that it was equaling an actual pull-up, but it had a little tension at the bottom of the pull and I figured I'd use it for a few weeks and maybe get a heavier one later on or on a repeat of the program.
Again, this wasn't a band-snap incident. This was the handle that cracked. By rotating that 1/2" crack away from the nylon fabric running through, my wife can still use it, but if I give it another go it'll probably crack completely and be useless.