I'm doing Optifast, and doing very well on it. It's not cheap, though, and your mileage may vary. It is a very restrictive program, in that you only drink 5 shakes a day, and I believe it's only available by prescription. However, I'm the type of person who thrives when there are strict rules - if I cannot eat food, I won't; I just cannot cheat while on this program. Not only because all food is off-limits, but the shakes themselves are pretty expensive. The shakes don't taste that great, but you can add sugarfree syrups to them to make them taste better, and you're allowed to still have coffee, tea, gum, diet soda, basically anything zero calorie.
What I really like about my Optifast program is that the clinic I'm doing it in makes us have weekly group meetings with a counselor to discuss weight and diet issues, and you also weigh in (privately). It keeps you motivated and focused.
One word of advice, though, is that once you're off the program, like all diets, it's very easy to gain the weight back on. You have to be careful to maintain a healthy lifestyle.