I went to the one in LAX last April. I thought it was pretty good. I learned a lot in their workshops that they hold especially the CRM and interview one. You get to network with a lot of the airlines that are currently hiring or have plans to hire. And make sure you keep those contacts. Send them a postcard every now and then. In terms of getting a job at one of these fairs with my meager 1400 hours, forget it. But you do get to learn a lot in their workshops and meet lots of people looking for work that range from Civilian CFI's to F-16 pilots. As far as if its worth it I don't know it all depends on what your expectations are.
No you don't have to be a member of AIR inc to be there. You just have to pay them somewhere around $200 +/- it varies on how many weeks in advance you register for the seminar. I think the fair Yankee was talking about is in Washington D.C. around the 18-20th of Oct. But I'm not sure where in D.C. As far as Air Wis attending I don't know. But Alaska and surprisingly American Eagle (I don't know why) are always there. They probably have more info on their website.