Cons: - Not an established forum. Unknown quality and it seems to be just 1 guy working on it.
None of these 3 options make it so the search field in drupal pages also search the forums. Google could do this for you if everything's on the same domain, and apparently drupal has some hooks to make this easy.
Looking forward, the DruBB group is planning to improve the default forums in drupal. They've got their work cut out for them.
This research was done for a community/forum website I am helping setup. More info on that in the future.