I do PC in Australia - I'm in a very small club, and am one of the intructors, as well as the oldest rider. The bigger, 'fancier' clubs tend to have a lot of politics - You generally don't realise until you have been going for a while. Our club is small so we are like a family - All help each other, swap horses, decide what we want to do, etc.
It gives you a lot fo opportunities - Progressing through the grades, and also certificates that can verify your knowledge. I have my D, D*, C and C* certificate and am working on my K - the one mentioned above, I have done the 40km ride and am currently breaking a horse in. It's really nice to have a formal acknowledment of your knowledge/acheivements with horses.
So really - It depends on the club you join. They are all different, and have different focusses (disciplines). It is run by volunteers, so be prepared to help out - There is nothing worse than riders who join and then don't help or have parents who help!