We have had Passport America for a number of years. We typically make most use of it while traveling for our "over-night" stays along the way. As most have already said, it pays for itself easily every year just using it for the over-nights. We love it and renew it every year.
In one instance we were able to use the discount for an 18 day stay in South Texas (near San Antonio). Due to the off-season the campground allowed PA discount throughout our stay. I would encourage you to buy it and feel certain you too will renew every year. Have fun.