Mantis is an improvement over Bugzilla, for sure. However we saw huge performance problems once we hit the 50k issue level. It was also much more difficult to modify than Jira (which we moved to). Although I guess among free options Trac and Mantis would be my top two.

Mantis is okay - we used it for a (school) project last year, but for what we needed it for it was overkill.