My husband has it and was doing it for a little while. It is a VERY intense work out. In fact, it's such an intense/advanced work out that they don't expect you to be able to do all of it at first. Basically, you watch the DVD while you're working out and you do the best you can. You do as many reps as you safely can for each move and as you continue the program, you'll be able to do more and more.
Like with anything, if you stick to the program, it works. However, it's such a hard work out to follow that a lot of people might try it for a few days and end up so tired and sore that they're discouraged from ever doing it again. When my husband did it for 3 weeks straight, I could see (and feel!) a difference in his muscle definition. That was great, but then he quit using it :-(