I've done weight watchers twice and it really works. I lost about 3 stone with them by going to classes when I was about 13, then a couple years ago I lost around 3 stone but that time by doing it online.
I personally would recommend doing it online because it's cheaper and you can still get the support from their online forum. You can also look up points values online and plan your meals that way.
I found that I didn't get much for my £5 a time at the classes. The talks didn't really teach me anything so all I was really doing was paying £5 to be weighed!
The only thing to be careful of is because you can spend your points on any food you want there is a huge temptation to use them up with naughty things like choccy and crisp. Although you may be losing weight you may not be eating a balanced diet. I also found I was eating all there processed ready meals instead of cooking from scratch so the whole family ended up eating different meals!
You might also like to consider Scottish Slimmers, they are very similar (owned by slimming world I think) They also work ona points system but you get portions of fruit and dairy products as a freebie so help to encourage a more balanced diet.
Hope this helps!