I have done Insanity, it is brutal but so much fun. I was at target weight when I started it so my aim was to boost my fitness. I was already reasonably fit from years of running and Jillian Michaels dvds which was important when starting Insanity. My fitness did go through the roof and I felt amazing! It is vital that you eat well, no processed rubbish, you will need all the vitamins and minerals you can get. Also get as much sleep as possible, the body needs to recover. When working out remember that perfect form is key, rather than high reps or keeping up with Shaun.