I have thought about joining SMA in the past, but I have personally had good luck finding jobs through personal contacts/networking, the DC/Baltimore AEA liaison committee's postings, and the DC Theatre Tech pages (a yahoo group).
I have an SMA membership- although I don't find a lot of DC area work through it. I would agree that personal contacts and networking have been the top ways that I have found work in this area.
Benefits:
It's nice to get the job postings via email, and I find it more useful for out of town listings (including opera). I'm a fan of the Operation Observation, and hope to be taking advantage of it again when my schedule permits me. And they are organizing relevant online chats- today's is regarding tax preparation, which I think will definitely be interesting. So on the whole, I think that it's a good resource, and that it's working towards becoming more relevant and useful for its members.
That being said, it doesn't do much for me in terms of getting work and making connections in DC. Some of the other resources mentioned here (the DC theatre tech yahoo group, and the stage management forum being held next Monday) are more specific to the area. Many of the job postings are for showcase productions in New York- not something I'm interested in. I don't know that it would have done much for me when I was in college; there are a lot of other resources available, including job listings on Backstagejobs.com. And in terms of observations, you could contact some of the local theatres directly and ask to shadow someone if you were interested.
On a side note, someone on this thread mentioned ArtSearch- I think UMD has a subscription, so you can probably access it through them instead of having to pay for an individual subscription.
I weighed the cost vs benefits in college, and waited until I was a few years out of school to join.