I buy it when I know I'll use it. Last time was about six years ago when my father was staying at a Passport America campground and I was stopping to visit him for two nights. Only got back $32 and didn't find another campground that I wanted to go to so lost out on the other $12.
Having bought it again since I don't use the type campgrounds that are members. Would have stayed at one or two but you either had to make reservation, wouldn't be accepted on the nights or some other criteria that I didn't meet. For me with my traveling style and preferred campgrounds, I wouldn't get a return on my investment.
As with any membership, you need to make sure you can abide by the restrictions, get more return than investment, and know how to terminate the contract when no longer needed.
2015 Ram 3500 4x4 Crew Cab SRW 6.4 Hemi LB 3.73 (12.4 hand calc avg mpg after 92,000 miles with camper)
2004 Lance 815 (prev: 2004 FW 35'; 1994 TT 30'; Tents)