Recs.
Updated
SpecsUpdate
Pros
Pro Importer from many other forums
Vanilla Porter can import posts (comments), threads (discussions), users, categories, roles and passwords (hashed) from most other forum software: bbPress, Drupal, MyBB, NoteBB, phpBB, PunBB, SimpleMachines 1&2, Toast, UserVoice, vBulletin, Xenforo, YAF.NET and others. Some importers support avatars, attachments, signatures, and PMs. A few support tags and user notes.
Imports aren't flawless, but they do most of the work.
Cons
Con No post edit history
Old versions of a post are only visible to admins, which may lead to a credibility problem. The amount of time during which users can edit their posts is configurable from a few minutes to indefinitely.
Con Poor search, unless using Sphinx
The default search that comes with Vanilla produces poor results because it uses MySQL. One needs to investigate integrating with Sphinx for much better results.