I'm a member of a couple professional societies. Membership gets you some publications that are universally used in design as part of your membership fee every year. Also because my company pays for the memberships and it yields me one free dinner+drinks a month.
I might join the baltimore engineers' club in another year or two, if I can get in under the corporate membership. It's in a pretty good location in Baltimore and has pool tables, bars, wi-fi, etc. It would make a nice pit stop on my way to going out further up Charles St.