Ive been working with Bugzilla for over 5 years now, across 3 different companies. Ive never had to do the installation so I cant speak to its ease or difficulty. But the developers and sysadmins that did do the installation/maintenance never complained (not like they did when having to user Mercury Test Director).
It does have limitations, some mentioned above. Access control is not mature. But weve always dealt with that by expecting people do not do things they should not do. A feature to associate a fix with a build/release would be nice. But its free, fast, and easy to use.
If you have a budget for these kinds of tools and want something more polished you might check out [url=http://www.fogcreek.com/FogBugz/index.html:2q6r0jkx]FobBugz[/url:2q6r0jkx] by Fog Creek Software. I have never worked with it but it looks pretty slick from the web site.