Every time we move, we buy a membership to a local zoo. We go so often that it would be ridiculous not to. We chose the Dallas zoo over the Fort Worth zoo because it has reciprocity with cities all over the country, and we travel enough that was a good selling point.
The zoo itself is not quite as nice as the zoo in Fort Worth, but it still is very worthy in its own right. With membership you get free parking and rides on the carousel and monorail. The monorail is great, almost like going into a safari park.
Members also get invitations to parties and camps that aren't generally available. They recently had a birthday celebration for members only.
We're lucky in this area to have two very nice zoos - it's well worth your time to check them both out.