P90X is just a fad workout.. eventually it'll go the way of Tae Bo and 8-minute abs. It's probably a great workout if you eat right, and do everything the video instructs you to do. Unfortunately, a lot of people use it who are really too far out of shape for the program to really work. For some reason it's immensely popular, yet the people who claim they do it still have quite a bit of flab. To each their own I guess. I've found the best results with running and cardio and good old fashioned weight lifting. As to the original question, I rarely do pull ups to muscle failure. I do three sets of 8, the last set is a bit rough but I get it done. I don't know how many chin ups I could do, those are a LOT easier.