Kind of a mixed bag on Passport America:
On the positive side, the discount is large, generally 50%. Also on the positive side is that the PA network is relatively large.
On the negative side, these discounts are subject to blackouts and other time/date limitations, as the individual park owner sees fit.
In general, you'll find that all discount programs apply to the daily rate, not to the monthly rate. But that's ok. At most places that I'm familiar with, the monthly rate is already discounted more than 50% anyway.
A common question is whether something like PA is more valuable that, say, Good Sam. Tough one. On the one hand, the GS discount is very small, generally 10%. On the positive side, there are no blackout dates, and the network is also pretty large. On balance, then, it really depends on you and your schedule: If you pay attention and really take advantage of PA, then it's certainly worthwhile! But if you don't really work to take advantage of PA, then it's probably more worthwhile to do the Good Sam thing and receive only the tiny discount.
I'm sure others will provide info about other programs, something like Escapees, etc. In each case, evaluate on essentially the same basis: Size of discount; blackouts (if any); size of network.