Ages ago ActiveState started using bugzilla, but there was on primary problem with it. The usability and appearance was not what we considered to be polished. Yes, I'm being polite. At the time, we forked our own copy of bugzilla, had many patches to it to make it nice, add some features we wanted, etc. The problem with that: we could never update to a more recent bugzilla without a high amount of effort.
With the latest incarnations of bugzilla, it's feature set finally got to a place where updating became more efficient than trying to maintain a six year old forked version. We pulled the plug on our forked bugzilla and moved onto the current code base. Unfortunately, we lost our polish in the process. That has now been addressed, and we've got a whole new face for bugzilla. Our webops team, and Tara Gibbs specificly, has put in a lot of effort to put a pretty face and smoother functionality on a web property that is critical to our operation.
You can see the fruits of our efforts on bugs.activestate.com. We've still got some work to do but this is a big improvement over the default install.
As each improvement to bugzilla itself appears, especially in the area of modularity and extensibility, we'll be able to keep up. So I also have to give a big thanks to the bugzilla development team.