What are the pros and cons for using Drupal Advanced forum or phpBB?
demo of Drupal Advanced forum is here: http://socnet.shellmultimedia.com/forum
this demo used Blue Lagoon Stacked styling
how to style your advanced forum is here: http://drupal.org/node/234042

For a good example of using phpBB with Drupal 6, see here:
http://www.teenrc.ca/phpbbforum
you can download phpbb skins from the internet
Drupal has module for phpbb. drupal.org forums has a drupal 6 version

I like the style of phpbb for the posts with the author pane on the right hand side.