I use phpBB at the moment. It used to be pretty bad but version 3 is acceptable.
The 3 things I hate about phpBB are:
1) Lack of SEO URLs/ pretty URLs/ custom permalinks structures
2) There are no plugins. There are MODs – meaning installation is a pain in the back and code is changed. Forget about updations completely – it is excruciating. Contrasts with the rather pleasant one-click installs and upgrades that WordPress has made me accustomed to.
3) Lack of multilingual capability. WordPress has WPML…though I am getting increasingly disillusioned with it (critical features missing, basic things like Canadian flag, Slovak language etc missing, not using international conventions in naming, arrogant and unhelpful support etc). I run a multilingual site and need multilingual forums. If that’s not possible, then to begin with I need a Google Translate gadget so that people can read in multiple languages at least.
It’s not just the lack of these features that cheeses me off…it’s the unwillingness of the developer community to consider these for 3.1/3.2
And there is a major rewrite underway v4. I believe v4 will be the wipeout of phpBB…they are writing from the ground up, which is never a great idea (http://www.joelonsoftware.com/articles/fog0000000069.html)
So excellent opportunity for bbPress.
I would switch as soon as I see the aforementioned 3 in bbPress…