I've been a member of the Society of Children's Book Writers and Illustrators for five years now, and gotten all kinds of benefits from this organization. I found my original critique partners through SCBWI, and their conferences are consistently outstanding. Our local regional chapter also holds schmoozes, where we can meet in person to brag, gripe, and share information.
I highly recommend them, but I have run into one problem: one or more of my novels may end up in the New Adult genre rather than Young Adult--which is as high as SCBWI goes for age levels.
So, partly to cover all my bases (but mostly because I've heard great things about their workshops), I'm also joining Romance Writers of America. Hubby is giving me the membership as my Christmas present, and I'm so excited to see what the local RWA chapters have to offer! There are actually 2 chapters nearby, and I may end up going to both of them.
What about you? Do you belong to any worthwhile writers' organizations? Sisters in Crime is another one I've heard good things about; I've actually been to part of the Left Coast Crime conference, even though I don't write mysteries.