Our PassPort America membership has more than paid for itself in discounts. We use it primarily for stops in between our destinations (on our way from Point A to Point B) for example.
I've been interested in memberships like Coast to Coast and the like, but am wary of expensive "buy ins" and/or being stuck with yearly maintenance fees. (Someone on another website compared such memberships to STD's: Easy to get, painful while you have it and nearly impossible to get rid of).
I wish someone with vast knowledge and experience would write a primer on the subject.