I've been doing the Insanity program since April 2010 and I've recently started doing the Insanity: The Asylum program along with it. The results have been excellent and I love the programs.
In order to do well with these programs, you must stick with them, know your limits (but still pushing yourself, you'll gradually improve), and eat a healthy diet (I'm somewhat guilty here as I have a bad sweet tooth).
Also it's necessary to know your fitness level going in. If you're out of shape in the beginning, you may need to move a little slower until you're conditioned. Case in point, the first day I did the Insanity program, I almost passed out..after a while I can do a lot of the exercises with little or no rest (although Asylum is a different story
).