I would say that some of the Insanity workouts are equivalent to CCC and some are harder. None of the main Insanity cardio workouts are, in my opinion, easier than CCC.
With regard to your knee/back issue, if you think CCC is pushing your limits, then Insanity may not be for you. But if you like CCC and incorporate it periodically into your workout regime without negative impact on your body, then you might enjoy the Insanity workouts.
I've done a full Insanity rotation, but like Dela, I think the Insanity workouts are great for incorporating in with another regime. In other words, they don't have to be done everyday. If you like to push yourself a couple times a week and are looking for some of the toughest cardio DVDs on sale today (at least in my opinion), then I would encourage you to buy Insanity. I love these workouts, and on those couple days a week when I really want to push my limits, I always reach for them.
Hope that helps!