I finished Insanity and have to say that for anyone to say that these workout programs are a waste of money, that's just not the case. It's all about what's worth it to each individual, what makes it easy for someone to stick to a program. For me, I got Insanity because my girlfriend and I were always laughing at the cheesy infomercial, but then the more I looked at the workouts, the more they looked like very good modifications on a series of workouts I did with a personal trainer a year or so ago that involved no weights or anything. I loved the idea of that simplicity, so I went searching and found someone local selling the DVD set for just 50 bucks.
After completing the nine-week program, I recommend it wholeheartedly. I loved the simplicity of just doing something at home when I had a spare hour rather than having to go out to the gym for the usual wait-for-the-open-machine routine, and having a calendar regimen/workout schedule to stick to kept me far more disciplined than going to the gym usually does. The end result: lost weight, added muscle (especially in my legs) and a far improved overall fitness level that has given me a quicker first step and much greater endurance. By no coincidence, my tennis also dramatically improved and I even just won my first league title.
Insanity is fantastic for tennis, because it truly works the leg muscles more so than anything else and also puts an emphasis on body control and balance, which are two key bases for tennis and areas of my fitness I'd neglected before.
Now, working in tennis WHILE doing Insanity simultaneously was tricky. There were times when I felt absolutely taxed in a third set (if I'd worked out the same day) but I remained careful -- making sure I had good shoes and, most importantly, was extremely careful about nutrition and giving myself plenty of healthy calories to fuel myself. (once I started concentrating on nutrition, my energy level spiked upward to match the craziness)
Anyway, just some thoughts on Insanity. Full-on recommendation here. It's all about finding what workout program works for you. Insanity may not be for everyone, but for me it was great. I'll probably start another modified round of it soon.