A month ago I received a request from the University of Coimbra to look into the current options of Java based web forum software to provide my opinion on each of them.
To be honest, I am not impressed nor convinced that any of them are suited to power a popular web site that gathers a lot of (malicious) attention.
Regardless of personal preferences, I am sharing my opinions and impressions after evaluating each product that might help other people in the situation of choosing a Java based forum. You find this report at http://goo.gl/TfvKn
My ranking for these products is the following:
1 - JForum
2 - mvnForum
3 - JavaBB
4 - Yazd
5 - JSforum
6 - JGossip
In the forum arena, I should really note that PHP based forum software from groups such as vBulletin, IPB and myBB will really save you a lot of headaches in the future. Plus, it is possible to run PHP inside Java using a framework such as Quercus PHP.