Our local school districts both do this. They have speakers come in and give presentations once every other month or so. I've been to a few of the presentations. I have found some of the speakers to be useful but haven't found the group to be something that is of huge benefit in terms of social networking with other parents.
I think that the main issue here in terms of it being a good support organization for parents is the way that our local districts ID gifted. Even the GT coordinator at one of the schools has said to me that we ID more high achievers than gifted kids and, thus, these events draw more parents of bright high achievers than parents of gifted kids. That makes it such that one can find some parents who are great to talk with and who are dealing with similar issues, but also such that many of the parents with whom you would be talking at these events aren't the best for commiseration or idea bouncing in that they don't have gifted kids.
Sorry about the run-on sentence!