I sorta migrated my board users to phpBB2. Mostly the only reason is that I can more easily integrate the board into my site because of the database and such.
Also because my site was php and the board was cgi, the whole thing sorta made sense. I do miss ubb though. phpBB2 doesn't have as nice of a control panel and the features are limited. It's freeware, so what can you expect.
It also cuts the server load which helps my host who is a friend of mine.