my point of view:
From running wwwthreads for about 1 1/2 years my users love it.
They came from a UBB forum and they all say they love the wwwthreads forum.
Style sheets are cool as you can set up different ones and the visitors can pick the one they like. I hope to work on a way to allow users to upload their own styles sheets when Scream gets the PHP version done. (Since I am a PHP programmer and not a perl guy)
You can NOT beat the supportive community here and Scream is great and swiftly fixes any problem you run into. (which are very few)
http://www.clickteam.com
