I just joined one here in Houston, and I've been trying to figure out when to attend one since they meet for different reasons and the ones I wanted to go to, so far, haven't been good dates. But I do plan to partipate.
I've particpated on a MeetUp on publishing a couple of years back and that was very insightful, but they stopped meeting. Also, people who were there to sale their services started to make up most of the group, so it messed up the atmosphere.
But I think they're awesome! Especially if you're in a big city. In smaller areas, there aren't much groups to particpate in anything. I think I'll participate in a Meetup that go to movies!