Short version: you can have up to three road service calls a year. Towing up to 3 miles unless you get the "Plus" membership (costs more) which is like 100 miles if necessary. If you have the basic plan you can get towed more than 3 miles but you have to pay for the extra miles.
Other than that, you can use them for vehicle title transfers, driver license renewals, buying travelers checks, free maps, free TourBooks (which list places to stay/things to see). They also have a travel agency and sell insurance but I've never checked those out.
They also have a program called "Show Your Card and Save" which gives you discounts at different places. For me, their LensCrafters discount just about covers my whole membership fee when I get new glasses.
I think it's worth it, but you need to make sure you understand the limits of coverage for road service if that's your primary motivation.