I look at all workouts like this:
I start out doing a little bit, as much as my fitness level allows me to do safely and effectively and within my limits. As I do the workout more often, my ability, fitness level and skill improve and, before long, I'm doing the whole thing and saying, "Wow, that was easy!" At that point, it's time to move on or increase intensity.
I think the Insanity series, STS, Shock Cardio, etc are the same. If anyone can put any one of those DVDs in, do the workout and say, "Wow, that was easy," then they have nothing to work toward or improve on! I'm totally sure everyone who has bought the Insanity series is on a learning/improvement scale and it will take time to get to where they can do the whole thing! (MHO) Any product that isn't challenging, isn't worth buying! We have to challenge ourselves constantly to make the gains and reach the goals we are all seeking. These goals are different for each and every one of us and some of us look at Insanity and go - whoa, not for me, no way! Others look at it and go - I can't wait to do THAT!
I also agree that any workout can be unsafe for someone who is not ready for it. It's a matter of fitness level, skill and core strength. Me, I can hurt myself just walking!
I'm graceful that way! But, I keep going and participate in anything I want to try and any workout I want to challenge myself with.
Anyway, my point, really (I do have problems making a point sometimes - sorry!), is Insanity isn't for everyone!