I run a UBB with about 350 members (right now) and some of our topics go into the thousands of posts.
There is great degradation of speed on the UBB in the larger topics. UBB generates both an HTML and a CGI file for every page of every forum, so it uses a LOT of server space rather quickly.
We ran a BETA test on WWWThreads 4.3 and found the performance to be outstanding and the features to be wonderful. The reason we didn't buy it was that we couldn't control the number of posts per page, and having to wait on 2k+ posts to load on 1 page would be detrimental to us.
Rick has told me that they are going to have a user option to control the number of posts per page in the upcoming 5.0 release, so we plan on purchasing it for our next project.
If there was a conversion built in for UBB -> WWWThreads, I would definitely consider switching our current forum to wwwthreads, but we would lose about 20k posts since there is no conversion routine available.
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