I've done some phpBB2 installs, and useability wise it's pretty good.
Security is something I worry about however. A while back I disclosed some vulns on their forums with fixes after they wouldn't respond to my emails, and their response was to flame me for (get this) not attempting to contact them and ban me from their forums. Thankfully they at least fixed the bug. I have to wonder though, if that was the response I got then, whether they take a serious stance on security. The way I look at it, all the forums I'm running it on are piddly little forums that aren't in any way connected with money, so I'm not all that concerned.