I'm just about to complete my 4th week of Insanity. It's super intense, but it's awesome. I already see results (I have completed Turbo Jam a few times and Turbo Fire, so I was prepared for cardio). You use your own body weight for 'weight' training, so you don't get the bulky arms.
I already see results, and I'm down about 8 lbs. Which is not bad considering that I was already after other workouts..... I can post my results once I complete the first section (4 weeks)... Good luck- I think it's worth every penny!