I did Insanity last year around the same time as Eric. I wasn't fat or out of shape, but the workout will put you in your place if you "think" you're in good shape. It was about 4-5 workouts before I could finish one. Once I could do a complete one I began the actual program. At the end I was doing the max workouts at the same pace as the folks on the DVD.
That said, I was dissapointed with the results. I didn't get the DVDs to lose weight. When I started I was at 185. When I finished I was at 185. I got the DVDs to tone and get some muscle definition. Didn't really happen for me. I had a before pic, and there's no real change like you see in the ads.
Best results for muscle and definition have come with kettlebells for me.