Weight Watchers is great because it puts the onus of responsibility on the dieter. Nothing is strictly off-limits, but knowing you have to stay within your points makes you start thinking in terms of choices, knowing that you can have the brownie OR the cake OR the hamburger, but not all of them today. I have known many people who lost weight on WW because even though they knew sweets and junk were bad for them, they didn't realize HOW bad because they had never actually measured the serving sizes before.
The downside is that it can be hard to obtain points values for certain things, and it can be fussy, as another poster pointed out. I am more of a control freak, so I like tracking calories, fiber, fat, etc on my own. Furthermore, I am cutting carbs because of endocrine issues of my own, so the WW tendency to skew in favor of lower fat/protein, higher carb foods doesn't work for me.
All in all, you have to figure out what works for you. If you find it too cumbersome to obtain and track the points values, then in the end it's not suiting you.