I did Celebrity Slim at the start of the year and found it useful for the first 2 weeks or so.
It basically is a low carb/low calorie diet, of which I think you could do on your own and do it a healthier, I ditched CS after a month and just watched what i ate, upped my veggie intake and salad intake, stopped eating bread, rice, pasta and potatoes etc. CS still has a lot of sugar in it and really, you could have a healthy big salad for lunch instead of a shake for lunch and you'd find that the Salad has less carbs, calories, sugar and is more filling.
Tony Ferguson and Kate Morgan seem to be a bit like CS, but I know on both of TF and KM you can eat more on it, even Betty Baxter lets you eat more but I've heard people say the shakes are gross. (I don't know if that's true though). CS seems to be a bit extreme and the information i've found on it varies. For eg Cat (the girl from the Biggest Loser who promotes it) said on the DVD that comes with the Starter Pack, said she eats a shake for Brekkie, a shake with an apple for lunch and then dinner would be protein and veggies, but in the Priceline magazine that was promoting CS, they did an interview with her and she said that she eats more than that, like soup and salad and stuff.
I think it's good that you are giving it a go, but at the same time, unless you are learning new eating habits and adopting a healthier a lifestyle, you'll probably find any weight you lose will go back on. I don't think you should have 3 shakes a day, you need to think about what you will be able to maintain in the future. It's all well and good to lose weight and be thin, but once you're thin you have to maintain it, you can't just go back to the eating habits that got you big in the first place. Believe me, I've gone up and down for years and finally understand the importance of maintaining!!
Good luck with it