Been doing insanity and insanity max 30 for nearly 12 months. I switch between the 2 so my it makes it harder for my body to resist against what I'm doing. No doubt it's great for blitzing fat and adding shape to your body.
The main 3 benefits for me have been fitness as you hardly have a rest, good abs as doing it with a decent diet reduces body fat, and really defined legs due to the shit load and variations of squats and generally jumping around.
Saying that, I haven't built the muscle I used to build via weights due to it being bodyweight exercises. You'll shape your pecs and add some mass but not much. Your legs will shape well but won't be very muscular. Surprisingly you add decent size and muscle to your arms and shoulders the strength exercises requires most of your bodyweight pressuring these parts of your body.