I'm in week 4 of Insanity right now and I'm enjoying it. I completed my first round of P90X last March and did the workouts a couple of times a week throughout the rest of the year. Insanity is tough. It's intense cardio every day and you have to be mentally prepared for that. I know that the Plyo day in P90X was fairly intimidating even when I got into pretty good shape. Insanity is like that every day (with the exception of your 4th day which is a light workout recovery day).
You see quick results with Insanity. I was already in pretty good shape when I started it and I can see some changes already. It's not a great muscle builder for your upper body, but I don't think you'll lose much either. You do a ton of core work, push-ups, and holding plank position for various things. You also do some tricep and shoulder work. It's amazing how tough 8 push-ups can be after you've put in about 30 minutes of killer cardio....
The cardio abs DVD is the best of the bunch. It's awesome--minimal pain and great results. I loved P90X and would recommend that to anyone, regardless of your current level of fitness. However, I like the cardio abs much better than ab ripper x.
Overall, Insanity is tougher to just jump into than P90X. You can burn a ton of calories by lifting weights with a couple of cardio days mixed in. P90X is very effective. If you're not in decent shape, you will struggle with Insanity. Intense cardio everyday is tough on you mentally and physically. However, if you can stick with it and listen when you body tells you to stop, you'll be happy with the results.