There were a lot of pros and cons on turias` homepage on Blaine`s forum and phpbb2. As for me I like to have everything in one place like GUS, Badbehavior etc.
If you have a huge forum you may want to look into various systems to compare but usually the dreams are much bigger than reality.
I just started using the forum on one site. I only seem to have few problems especially with internationalisation:
http://www.portalparts.com/forum/viewtopic.php?showtopic=9875
As far as I remember the forum of glfusion is different from the normal forum plugin.