I did Insanity in the past under three different conditions:
1) First real workout program ever. I decided I wanted to cut, so I kept cals around 1800-2000. I definitely got fit, and to this day I don't think I've every been so lean. Six pack and everything. Cardio through the roof. My strength was pretty low, though, but I gained it back quickly. I was around 22.
2) After lifting for a year and a half, I ran the program on a maintenance diet. I got pretty fit, my numbers on the fit test were better than they were the first time around, and I think I looked the best I ever did after I finished the program. Not the leanest, but this was my prime, I think. I was around 24.
3) After lifting a lot, and a lot, and gaining a bunch of weight and strength, I did Insanity on a cutting. I did lose a lot, but I didn't feel that I looked that great at the end. Granted, I was a little inconsistent and had sloppy days with my diet. I did lose a lot of weight and was able to fit into a lot of old clothes. Didn't quite make it back to where I was, however. I was 27.
4) Now, I'm basically where I was before (2). I'm trying to eat at maintenance. I hope I can get into the shape I was in at 24. That was awesome.