In reply to kev74: I'm not someone who likes exercise for its own sake. I'd rather ride a mountain bike through a 100 miles of mud than ride an exercise bike for 5 minutes, rather play any sport than go to the gym, rather take the dog out for hours and miles than jog solo for half a mile, I stay in shape by chasing my dog through Epping Forest on my bike and climbing when I can. The daily 1 hour power walk probably helps too.
but ...
My wife likes to work out and I recently kept her company through a cycle of P90X, she had considered Insanity as well but it looked a bit high-impact and too many years of hurdling in her youth have left her careful of knees and ankles. I was already in decent shape before we started but after the 90 days, the most profound effect on me was flexibility and core stability. There was a yoga component to the workout that made me whimper but I think this was where the best gains came from. I'm actually thinking of doing another 90 day cycle just to improve my climbing ...