I have been doing both P90X and Insanity as well, so I can answer as well without highjacking the thread I guess
What is good about them?
I love the fact you can do these programs at home wih minimal equipment and space required.
For Insanity you'll only need a waterbottle, towel (because you're gonna sweat!) and your own body.
P90x uses a bit more equipment: pull-up bar, freeweights or restistance bands (the bands can be used for the pull-up excercises as well) and push-up bars (optional). Waterbottle and towel also needed offcourse.
The fact that you can workout at home takes away an excuse (not wanting to go the gym because of the wheather, busy hours at the gym etc) and I've found it easier to sqeeze in a workout without to much planning, just put on your workout kit and go if you have an hour to spare.
The workouts themselves can be modified and this is shown on the videos, so you can participate even when you're not a very advanced level of fitness.
Both trainers in the videos do a good job to keep you motivated. Tony (P90X) is kind of a nutter at times, and likes to have fun during workouts. Shaun T (Insanity) is a bit less playfull, but far form a drill sergeant.
The reason why I didn't stick to one program? One reason: variety. Allthough both programs contain enough different workouts to keep it interesting during the week, after you've completed the program (P90X 90 days, Insanity 60 days) you can pretty much recite everything that is said during a specific workout.
Also both programs have a different focus on different parts of your body. Insanity is (mostly) core work in lots of intervaltraining, where P90X also targets the legs, arms, shoulders and back specifically (besides the core offcourse).
By doing both programs I felt that P90X did more for my arms shoulders and back and Insanity changed my core the most.
Starting in a few weeks time I'm gonna do a hybrid of P90X and Insanity, to get the best of both worlds
Can't say how it compares to a regular class and personal trainer as I never did that (when I had a mambership at the gym, I did mostly cardiowork).