I run many forums successfully on phpbb.

http://usalug.org
http://opensuse.us
http://bashscripts.org

all run phpbb and have been for several years. I debated on going to a non-free option, but it kind of went against the grain for me. Many boards do use vbulletin, but the largest forum in the world uses a hacked phpBB........ so it ain't all bad

My backups are ran 4 times per day, and a total reload from backup can be done in less than an hour with mods completed over a span of a couple days.....because I do them a few at a time......the mods i mean.

I've modded phpBB quite alot over the years, and I guess it's what you get used to I find it easier to mess with phpBB than the others...... and I've tried most of em. So, don't discount phpBB altogether, it's free..... a definate plus !