I have been a member of the SMA since grad school, about ten years ago now.
I think it's funny to see people ask is it worth it to join. Like any sort of membership or professional group organization, a lot of what you get out of it is going to be what you put into it. You can join SMNetowrk, for example, not post, not log in, not take advantage of the site, and thus not get anything out of it. It's a group of people, with a common interest, and you can take advantage of what they have to offer.
I can say, I have found a couple of jobs from their postings (all of them have been last minute replacement jobs for shows that either fell through or closed early). I have found a lot of stage managers from the SMA. I have made contacts with a lot of other stage managers, and have networked to a certain extent (since I am pretty much attached to one theater, I don't network for jobs per se, just sort of network for networking sake).
The SMA is actually working very hard (I was a secretary for the organization for awhile back before my current work situation blew up . . . long story there) to reach out to regional stage managers after facing the rap of being New York based for so long, and they are constantly looking for new ways to do. (they have sponsored a SMA drink night or two in the DC area) Join and be active in the organization, if you are the joining type.