I did a few weeks of insanity with my sister, we were training together, but she got injured, so we switched to something else (moksha yoga) for a while. We're back to running now, and I'm doing some other DVDs. But I haven't forgotten about the Insanity program- I still want to do it. I'm traveling a lot the next 3 months so I'll probably wait until my travel schedule is clear before restarting.
I did 3 weeks, perfectly according to the schedule. The biggest problem I had with it was that my calves were killing me. It's a lot of plyometrics every day, make sure you have great shoes.