I finished Insanity a few weeks ago. It was the most difficult thing I've done exercise wise. I've done the gym, running, 10k Marine mud runs, crossfit/bootcamp and Insanity was just, well; its insane.
Its amazing how your body adapts to it, month 1 to month 2 is a huge jump. I was already in decent shape prior to starting because I was still doing weights and running. I am roughly 6'0" and 188lb, it took me a bit longer to finish but due to a lot traveling and injuries (not from the video's, went running with sis on an SF trip and inflammed my ankle tendons) but I have noticed some significant changes in my body. I will admit i missed about 3 videos month one (only cardio recovery days) and didn't do the max abs on month 2 but still did every other work out.
Prior to Insanity I could see my abs but after they are more defined, as with my chest and my oblique's are toned like never before. My weight is pretty much the same, I eat healthy but am not super diligent with it and as I said my weight was pretty much the same but my body became more defined.
I'm now back to weights and am using Insanity about 4x a week to keep up my cardio. I'd recomend it to anyone who is looking to make a change. I will warn anyone who is thinking about it and tell you it is extreamly demanding and will push you like never before. As I've said I was in good/very good shape prior to starting and it worked me. But if you have the motivation I highly recommend it.
My favorite vids
Cardio core and recovery (i do this on light days)
Vids I'm afraid of still.