I think the key to successfully using discount clubs like Passport America is being able to camp during the week (not weekends) and not during high usage times like holidays. If you are a weekend warrior, then that 1800+ of campgrounds they boast, will actually probably dwindle down to around 100 or so and then hopefully one that is worthwhile will be nearby.
But if I were retired and/or able to go during the week, I definitely would purchase it and use it. Also, it might be worthwhile to see if it will work for you for a week or two vacation.