We use Mantis. It's solid, easy to setup, it simply works. I'd prefer GUI frontend, but web gives us platform independence (win, linux, os x).

I think what's important are attributes that you can setup for a bug entry, and tracking the discussion around a bug (web, emails). I would love to have a bug tracking integrated with my editor and version system (CVS), unfortunatelly I don't know about such product yet.

It would be nice to see what was changed in between two versions, which bugs have been fixed, which occured, and direct links to files that were changed. Now we write this info manualy to mantis entries, but it's a pain.