Have to say I haven't - at least not so far. Though I did purchase a Weight Watchers home exercise dvd recently, and have to say I'm very pleased with the results so far after doing some of them for only about a couple of weeks. It's definitely got me considering using their online service at some point in the future - as so far I'm pretty impressed.
My Mum has gone to meetings. I think it is one of the better diets. It seems to be based on reducing calorie intake and only eating a certain number of points (other diets are more about changing the proportions of foods).
In terms of the meetings, my Mum just queued up in a long line and there were tables with books and products, then she got weighed. I think there were a few questions to fill in the paperwork. There were chairs set out and I think you could stay to listen to talks. It didn't seem that scary.
If you did considering going, you could email the person in charge for information and say you are nervous and ask about what happens at a meeting.