I use it for our guild forums. So far we've gone through the BBpress and Simple:Press as options. The former was too light and the latter was bloated and ugly. Apparently simple:press now charge for access to their support forums.
We've been using Mingle since May and although Mingle Forums can be a bit light and doesn't offer all the customisation that we'd want ideally for our niche use of it, I've found it to be reliable and we like it enough to start skinning it to match the rest of our wordpress. While admittedly not the most intuitive of forums that I've used (at least as a visitor), its caused me the least BS as an admin, and so I don't mind compiling a little hints and tips section for our members who want to get the most out of it.
With the PM plugin also made by the developer, we've found the forums to be a happy medium. While sometimes I entertain thoughts of attempting to bridge PHPBB for a more complete and integrated package, I don't think the wall next to me can stand too many more blows, nor could my forehead.
I did find youtube instructions for the Mingle Forums and they've been helpful. Maybe the reason why the forums appear blank is because the site admins have hidden the shortcode on a private page. We used to do that till we bit the bullet and decided to add members manually to usergroups to allow better access control.
I look forward to the developers next forum plugin and what I've read it's going to go further than Mingle in functionality. A few searches indicate it's been under development for a long time, but I think the person is a one man army.