Not very professional, but there are some nice people. They know what theyre talking about for the most part.
They use awful forum software, but the community is very professional and well informed. This is a very FCP-specific board, but you can get info about other stuff.
There is also the DV.com forum, but there are more high school nerds there than pros, and most of the folks are really obnoxious (save for a few). I have found it incredibly difficult to get useful information there.