I haven't tried it with any other extensions, so my rating is based on my personal experience with PHPBB3 only, so if you want to avoid a complicated bridge between Joomla! and PHPBB3, I can highly recommend JFusion to link the two user databases together.
Setting up requires you to make sure you understand the instructions very well - but we all do that anyway, don't we???
I will certainly use this again when developing other sites.