I like SMF. A lot.
Drupal's Forum module sucks fucking ass.
Far better to do what NHLA did, and use the plugin that syncs Drupal<->SMF accounts. It's quite nifty.
phpBB is fugly and has crummy APIs and a crummy (virtually nonexistent) extension mechanism.
As for SMF being "closed", yeah, but the APIs are well-documented and well-supported, and the extension mechanism is VERY wide-open. I'd like some more basic functionality APIs to be exposed, but that's picking nits. They chose not to release the source code, too bad. With the APIs as open as they are, it's better in some ways than GPL'd stuff. 99% of the time you want to add a feature, not plumb the undocumented depths of the implementation.