Are P90X or Insanity even worth doing if muscle gain is your goal? I feel like they aren't. I looked at doing P90X but ultimately decided against it because it seems to be more of a weight-loss / tone-up type of thing that would only add lean muscle instead of mass. I can't imagine that Insanity's any different. I'm already skinny as fuck with lean muscle, so a program like that is worthless to me. Plus, what happens when your 90 or 60 days are up? Either you jump right back in to it or you quit and start losing the progress you've made.
I feel like a well researched and self-designed fitness that's sustainable and modifiable over time will actually promote a more fit body in the long term than any of these 2 or 3 month programs will...
EDIT: I mean, if you just want to look good for a vacation or wedding or something than programs like P90X/Insanity might be of value, but for long-term sustained fitness and muscle gains, I feel like they're not the best way to go...