As far as i seen most of free forum software has a very slow development time. Same as for most of open source projects.

phpBB is crap in my opinion, and i tried to look myself for some good alternatives, but didn't find one until now.

Anyway, if you're looking for my opinion, vBulletin is also a bit crappy because their own architecture is somehow into 2002 ages, and also lot of things may be re-worked for giving a boost on speed (too much CPU consumption due to extensive use on MySQL).