I was with the PDSA for about 4 years, I took a 6 month career break which I saved for, but sadly turned into 2 year for family reasons - I couldn't work, then it took 2 years to build up the self employment enough to have a decent 'wage'. I found that if you pay anywhere near what things might cost, they get very funny with you. I paid £50 towards my Greyhound's dental because I wanted to pay as much as I could afford and their reaction was almost 'if you can afford to pay that much you don't belong here'. It was better when you could donate anonymously! In the end I just gave them a fiver like everyone else and put money in collection tins!
I also found that they were regularly an hour later with appointments, and although I was still entitled to treatment there up to April this year, I left about a year before that. I felt when you have to worry about fitting a 2 hour slot into your work day (because that's how long it took with travel) it was wrong to be there.
So, if you are entitled to treatment, and you need it, then take up their offer, but try not to stay longer than you need.
The one pedigree rule is a sham. If you get a second mongrel while you are registered with them, they'll allow that one. Yet my view is, if you can't afford the first one, for whatever reason, you shouldn't be entitled to a second. And I often saw people in there with second or third dogs. If they really wanted to save money they would only accept animals that already belonged to owners when they signed up. Sorry for the rant...