Thing is, every school and every PSA is different. And in fact, they change every few years as parents leave the school, or new staff come in, etc.
Ours: an Osted 'excellent' school so everyone assumed we had a big active PSA. Reality was, we had the bare basics (literally just the office roles filled by random people, all new to the PSA, after the previous more cliquey committee quit en masse). We managed to keep things ticking over for a year or two, just running discos, couldn't manage to staff a summer fair (ZERO support from the school even for one event. You'd think they'd want to have one event a year, but the head was on track for a promotion and her well-repeated line was "no one is interested in summer fairs anymore."
)
There's now a new head, one more amenable to supporting the PSA's attempts to have a few events a year. I can't stress how FEW events I mean. A handful.
Then down the road, there's a friend's village school. Their school PSA sounds like the Stepford wives, they are asked to bring contributions in every other week for sales or breakfasts, etc etc. I am genuinely gobsmacked by the amount of work that's expected of all the parents there.
So basically, if parents from that type of school are in the thread, they will of course have the view that the PSA runs everything, are cliquey, and don't appreciate anything. If parents from my son's old school are here, they'd probably struggle to come up with anything we've ever done besides a Christmas fair last year and the discos. 
For my own part, though, I can say I enjoyed my time as treasurer. I met more people, enjoyed helping out, and learned a bit about the behind the scenes of the school. I'm not a good school gate mum. I'd recommend it to anyone.
But feel free to run a mile if they don't appreciate you!