I've lost 10kgs on weight watchers several years ago. It stayed off too, until I started ttc and all of the emotional eating that went with it....then pregnancies and kids...If I could afford it I would do it again, but unfortunately, $17 or whatever it is these days is a bit too exxy for us at the mo I have over 25kg to lose and I could do with some help!
I think it works because it is a realistic 'diet' which the whole family can have. There's no special requirements or anything, just sensible eating. And they also offer advice on the other aspects of weight management like exercise and the mental/emotional side of it too.