I'm not on South Beach anymore but I was on it for about 6 weeks at the beginning of the year. While I was completely faithful to it (I'm very type A personality), I did not lose weight but it did some VERY positive things for me.
The best thing it did for me was show me that I wasn't eating enough. If you don't eat enough you can't lose weight. If I had been overeating before I started it though, I believe I would have lost weight in a healthy way.
Another very positive thing it did was teach me how to make good food choices because even though I'm an intelligent person, I had developed some not so intelligent habits and attitudes about food. The book eliminated the guess work, you don't have to think, just do each step.
In short, I think South Beach is an excellent primer and that you are not wasting your money by purchasing the book. Some of the foods are expensive so you may have to modify it a little bit to fit your budget, though.