I have never been too impressed with Wordpress Forum Plug ins and I would never use one for a full solution, but recently I looked again as I was looking for something light, yet functional to use simple as a place to answer support questions.
I was going to do a full installation of V- Bulletin (which was too much), but then I found this:
It's actually a pretty decent plug in and it does exactly what I needed it for, something on a smaller scale to use as a place for support questions.
Works with Single User WP and MU
Installation wasn't bad (it's a big plug in), my issues came from my FTP program not uploading complete folders, but after I got that worked out, it seems like it will be perfect.
Just thought I'd share