Done it, stuck with it for the whole 63 days, I've surprised myself with that alone.
In my case bobmcfc I'd definitely say it was determination, and pretty unexpected determination at that, as before Insanity I had not been to the gym for 3 years and probably did a couple of hikes in The Lakes in the summer but that is it. It becomes easier, not just the exercises (well, not really the exercises at all - they get harder!) but the planning and eating to fuel your body to do it, and I'm now in the position where I would actually prefer to stick with some of the meal suggestions and routine. I will be doing another round of Max Cardio and Insane Abs this week, I am actually looking forward to it. Madness. I'm certainly not super fit even now, but compared to where I was 63 days ago I am a million times better off, from the way I feel to what I can actually accomplish.