I see some talking about Bloat. I've heard that a lot but my forums even with 700 concurrent members and heavily hacked is pretty fast. Yes, I have a dual Xeon dedicated with 2gigs of ram and dual scsi's but I also have two other vbulletin forums running each usually with between 20-75 people on average and a bunch of other sites with small forums.
I think PHPBB and the other free "project" forums appeal to the open source and coding community that doesn't like having to pay for anything. A close friend of mine talks smack about vBulletin all the time and is on the SMF (used to be YabbSe) "team". He is also an old school coder / "hacker" and for the longest time talked smack about my servers running cPanel and me running XP Pro at home. WELL, guess what... He discovered cPanel / WHM and hasn't looked back. He also now runs XP...
After lots of talk we put my hacked up vbulletin 3x system with about 350 concurrent connections (not who's online but active people) against one of his SMF forums with about 75 concurrents. Guess what? I had substantially better load times. His site is also on a similarly spec'd server.