We have membership in Passport America, Good Sam and KOA Value Kards. Other than Passport America, the rest typically offer a 10% discount which basically covers the taxes on each stay. I'd say that 80% of parks also offer the same 10% off for AAA, AARP and Military. Doesn't seem like much but obviously it simply depends on how often you pay for a RV site. In most cases 10 stays will recoup your cost with a 10% discount
Passport America offers a 50% discount (or close to it) but there are quite a few restrictions (Sunday thru Thursday only for example and many months of the year with no discount at some resorts). Most also limit the discount to 1 or 2 nights but we've seen exceptions to that rule as well. Come to think about it, the thing I most dislike about PA is that the rules and conditions are all over the map, but possibly that's why they can offer such a great discount.
The worthiness of each based on their cost of membership is simply a matter of doing the math. If you only camp a couple times a year, the cost of membership will be higher than the potential savings. In our case, we camp about 75 to 100 nights (average) a year so it's a no brainer.
Even with the severe restrictions, the Passport America membership usually pays for itself in a week if you simply stay at about 3 PA parks. The Good Sam membership also gives you a 10% discount at Camping World when you buy stuff so take that into account also. I hate Camping World but they are still a good source to find stuff (that WallyMart doesn't stock) when you need it.
The KOA Value Kard offers points for staying at KOAs as well as the 10% discount ... just like a hotel. You can exchange points for free nights. You also get additional points for making a reservation online.
It's also worth noting that you must mention your affiliation and or ask about what discounts are available when you check in.