RockinRobin is 112% right - it is about what plan you are willing to stick with.
Weight Watchers: I think is a great program that helps tons of people. It wasn't for me. I didn't like counting points. I didn't like weekly official weigh-ins and I didn't like the price tag. BUT that is were I first learned the idea of portion control.
Slim-Fast: I have dabbled in this, but I don't see it as a realistic long term thing.
Alli: I haven't tried... I am afraid of $#!&ing my pants.
Curves: My mom did Curves for years and loved it. The only drawback is that you can't increase weight/resistance as you get better. Curves is GREAT for starting out, but I am freakishly strong and don't get enough of a work out at Curves (my mom took me to try it a couple times)
Acai: I haven't tried. I imagine it is like most other supplements that work when you use them along with diet and exercise.
I have also done some calorie counting. I have been to see nutritionist. I have been to a psychiatrist for some therapy. Now that I am into a groove... I know what is an appropriate amount for me to eat in a day. I focus on getting in veggies everyday. I eat fruit. I drink water. I jog. I strength train. I do these things because they make me feel good and I am focusing on that. I have been practicing on stopping when I feel full. I eat a small snack when I am hungry. I don't eat when I am not. I limit my booze intake. I limit snacks and sweets.
It sounds like a lot, but when I first started (nine years ago) I was 250 lbs and I only started with switching from white to wheat bread and doing 10 mins a day on the elliptical. Little by little, I added more and more and learned what worked for me and what didn't work.