insanity definitely increases your (cardio) endurance!
i just finished the p90x plyometric workout dubbed by tony horton "the mother of all X workouts" and it was actually pretty easy! this goes to show how much insanity has increased my endurance and i know that if i didn't do insanity, that workout would have killed me.
it was ok; not as good as insanity but not bad either. i didn't need to take extra breaks like i do with insanity and my legs/ass weren't on fire as they usually are with insanity because it goes at a slower pace and not as intense. on a scale of 1-10 in intensity, it'd give it a 6.
just a FYI for those who are looking to do a decent workout without all the power moves. it will make you sweat because it's long but he shows you a modified version that is low impact for beginners.