I did it, and I'd say the name is pretty appropriate. It is VERY difficult. I would only try to tackle it if you are already at a fairly high fitness level.
Month 1 is very tough, but do-able (again, assuming you are at a pretty good level already). Month 2 nearly killed me. the workouts are brutal, and long. If you aren't already pretty fit, don't even think about it.
Positives, it really works. If you commit to 60 days of Insanity, don't miss any workouts, and maintain a reasonably good diet, you will DEFINITELY lose weight, and you will see significant changes in your body, especially in the abs/core. On day 60, I was unquestionably in the best shape of my life (at age 38)
Negatives, it is REALLY repetititve. There are really only 3 main workouts in each month, and you do them over and over. Plus, you do the same moves, or just slight variations on those moves, across all programs. Which is to say, it gets boring after a while. Also, I'm not the biggest fan of Shaun T's style, which is essentially to say "push, push, you can do it!" over and over again
Now that I have done it once (August>>October 2010), I use individual workouts from the program for my non-running cardio days, and use resistance workouts from either P90X or Tony Horton's One on One. This prevents the boredom of doing ONLY Insanity.
Bottom line, its a very effective program, but it really needs a lot more variety. I think it works better as individual workouts incorporated into a hybrid program, but I have to admit, finishing the 60 days was extremely satisfying.