I was in Pony Club for a few years when I first started riding. I learned SO MUCH. I would definitely encourage it if you want to learn.
As for the competitiveness and bad attitudes, there are those in every club. I remember doing camps and seeing another girl in tears because a mom was telling her she didn't go over a cross-country jump when everyone else saw it and it was videotaped...
BUT, if you can stay strong and confident and realize that though you may not own a horse or haven't been riding since you were 2, that you belong there because you love horses and want to learn, you may have an amazing time, and meet some great people!
I think it's always important to remember that if you do not own a horse, it doesn't mean you're any less than another rider. The love of horses and will to learn is what brings all horsepeople together :)