PPA is great if you fulltime - we saved over $500 last year. But we tend to wander, wait until the last minute, and only use them for a night or two when we need a laundry. We have found some really great parks through PPA though we have also landed in a few clunkers. We don't make advance reservations at PPA parks, we call them en route to see if they have space. We have used credit cards and cash.
You won't get PPA rates at many parks in Florida during snowbird season but if you head there any other time of year, they have some good parks in great locations, including the keys. The same is true of anywhere else - during high tourist season, chances are it will only work for week days, if at all.
If you use it for two nights, you have pretty much made your money back. We use our PPA way more often than we do our GS membership.