I originally chose it because it was then the best-of-class forums solution, and although it is still very good software, competition from entirely free, open source alternatives is driving Groupee to change their business model. At first they tried moving towards hosted forums and away from downloadable forum software. Now (writing this in April 2007) it seems they’re changing their model even further, to the point that I can’t really tell what they do or sell by looking at their website anymore. Although the continue to sell UBB.threads, there are no longer any obvious links to it from their site.
In March/April 2008 I replaced my own UBB.threads installs with a custom Rails application that integrates much better with the other parts of the site. I want a cleaner, better looking, more useful interface for my users, and I don’t want to have to worry about PHP security holes any more.