An integrated phpBB forum will never be as tightly integrated as a native Drupal forum within a wider Drupal site. But it may be the best solution for people who
- have a phpBB installation which has a lot of traffic and too much data to migrate efficiently (though the module above has been tried on fairly large phpBB boards),
- who, more often, just don't like the default Drupal forum and prefer the feature set offered by phpBB.
See the integrated phpbb project home page here:
See also phpbb, a Drupal group on phpBB integration.