Hi,
I've completed 1 round of Insanity. I modified like crazy and made sure to keep my knees soft and posture straight. Don't be afraid to make some of the moves easier by modifying making it lower impact. I'd suggest warming up 15-20 Minutes before starting any Insanity workout. The warmup period in Insanity is super high impact and not what I'd call a warm up. Just really listen to your body if your knees or back are killing you then definitely modify and make it lower impact. No matter what definitely warm up 15-20 Minutes prior to beginning any insanity workout, your body will thank you for it and greatly reduce chance of injury and make the workout more fun. I really enjoyed my experience with insanity but I did my own warm up on the treadmill jogging for about 20 minutes first, and I also modified as needed and always protected my knees by keeping them slightly bent. I think you'll find this useful advice in enjoying your insanity experience.
Have fun and good luck!
Nora