I just finished my first week of Insanity, this is my day of rest. I consider myself in decent shape, I've done P90x but never felt I've gotten enough cardio and could stand to lose some winter weight.
If you're motivated and can push yourself, it will kick your ass. The first 4 weeks are "easier" and I've been left gasping for air by the end. I haven't made it through a whole workout without stopping at least a few times. It's amazing how much progress I made in one week though. You do the same DVD on Day 2 and Day 6. Even though I was exhausted from the whole week, I could go a lot longer already the second time around.
You do the same 5 workouts for the first 4 weeks (it mixes up the days), then have a recovery week during week 5. You still workout that week, but apparently they are easier workouts. I'm interested to see what week 6 brings as you start a whole new series but everything starts with the word "Max". I can't imagine doing anything harder then I am already doing.
From reading the forums, a lot of people go through the 9 weeks of Insanity to tone up, then start a combination of P90x and Insanity. Insanity does mix in enough upper and lower body to work those muscles, but it really is cardio focused so no one should expect huge gains from it. I can already see more definition though.
The key is you can't stop when your feeling tired or let up, you really need to push it to your limits so it takes a lot of discipline. If you don't have that, then I can't see it being any better then regular exercise.
One more note, it's high impact so anyone with foot/leg/back issues it probably isn't for you.