I belong to AIGA. It's a good resource and usually the local events are interesting and fun. It can be a good way to connect with other designers or interesting art/design figures. However, if you're more then 20 miles outside the "city" where the chapter is located, your event attendance will probably drop as not all events are worth the trip (I'm not driving an hour and a half for basically a visit to a bar/pub!) But in general the membership fees have been worth the membership benefits (professional discounts, access to job boards, etc.) One issue AIGA is currently facing is how to better include web design and web developers into the organization. Also they sometimes focus too much on international design collaborations to the seeming expense of local design communities. But they do a good job of promoting design as a profession worthy of respect.

I would also look at Meetup.com if you're aiming for something a little more ad-hoc and "local".

I also belong to AIGA. It is a great resource.

If I had the resources, I'd join SoTA as well.