When I joined, I received a lot of frequent flier miles and some good coupons. That made it worth the membership fee.
Since then, I've used their zone fare a couple of times, but usually I can find lower fares elsewhere. The system was very cumbersome. First I had to purchase the zone fare coupons and wait for them to arrive in the mail. Then I had to call to make my reservation. The seats are capacity controlled, like frequent flier seats but not as limited.
I think they've changed the system a little. I don't think you have to wait for the coupons to arrive in the mail anymore. But you still have to call to make reservations and maybe also pay a fee for using the phone service.
If you can get good bonuses for joining and/or you travel on routes where the zone fares have a price advantage, then it might be worth joining. Otherwise IMHO it isn't a very good program.