we belong to GoodSam. the 10% discount on camping fees easily covers the cost of the annual membership. others will recommend PassPort America for it's 50% discount. GoodSam has thousands of member parks, PPA has hundreds. and many of those parks have restrictions on when the discount can apply. KOA is not a club...it's a chain of parks, most of them nice...and pricy parks. their card will get you 10% off camping fees. we have one since we use one of their parks each year for a week and the discount at that park is more than the cost of the card.
how are you going to travel? if you stay in full service parks then one of the above cards will come in handy. the GS card will be good at many-to-most of the cards you'll encounter. we've actually run into parks that will give us the GS discount without showing the card although I suspect some of those are not applying a genuine discount. if you don't care that you may have to drive out of your way to get to a PPA park and you can live with the restrictions then PPA may be for you. if you'll stay at KOAs exclusively then you'll save a ton with their card. if you plan to mostly use public campgrounds (local, county, state or federal) then none of those cards will help.