[QuercusMax]

I've used (and customized) Bugzilla, used Google Buganizer extensively, used Jira for a year and a half, and also built an internal system consisting of a bugtracker + requirements manager + sprint planner + customer management system + manual test tracking tool + knowledge base.

Bugzilla was fine to hack a few extra fields into, but I wouldn't want to build anything around it. Buganizer was actually pretty nice, but suffered from too many competing tools built around it, most of which were just somebody's 20% project, so they kept getting abandoned. Jira wouldn't be so bad if it weren't so slow and annoying to use; only our TPM can keep track of how everything is set up.

The internal system I built was very specialized to our use-cases; it started out as a simple task list and eventually grew into a huge beast. By far the worst part of the system was the manual-test-management system, but that was just a mess due to its very nature. We were able to be very efficient with some of the custom functionality we made.