Yep, that's the true key. The replacement diets will help you drop and drop fast, but you do need to then have a completely changed way of eating or you won't keep the weight off. Same as anything. If you change your eating habits through Weight Watchers, and then return to old "bad" habits you'll gain.

I know for me it's more significant what I eat than (within reason) how much. I can eat my weight in seafood, no problem. 1/2 cup of pasta and I'm an oinker...