It is as hardcore as you think lol. I currently do this and im on my recovery week at the moment (so week 5). Next week im gonna be starting phase 2. I used to do the workouts before and mixed them up with P90x and TurboFire, so not really following the plan (but still getting a good workout), but in mid march i decided to follow the calendar.
The recovery week is no joke either and as i type my shoulders are aching, so are my hips lol. It is definately a fat stripper, and your heart rate is constantly up. If you're someone who tends to take long breaks, then this wouldn't really be for you, as the thing about Insanity is you exercise for long, then take short breaks. I have to say Insanity is by far the hardest workout i've ever done (and i've done the likes of 30day shred, P90x, TurboFire, Chalean Extreme etc.) I think this is mainly due to you needing a lot of endurance and stamina. Determination also plays a key role as well as nutrition. If you have knee problems, then it's really risky, as there's a lot of jumping involved that play a key part in the warm ups and the different sets. Also, unlike P90x there are no 'beginner' type alternatives so what you see is what you do. I think because of how insane and intense it is, you should look at what the fit test consists of as all of those moves are included in the workouts. If you think you can do it, then by all means go for it!
I haven't seen any visible results yet though i haven't really looked too closely and im told the visible results will come during the second phase, but i have had alright losses on the scale, and my body is definately stronger.
Good luck with whatever you decide x