Trac is an issue tracking system that provides a tickets, wiki, timeline,roadmap, and svn (and other cms) integration. It is a system that is more comparable to in scope to redmine than the ~pure bug tracker candidates like roundup, bugzilla, or mantis.
From my recollection of history (possibly distorted) - Trac emerged as one of the first integrated open project management web applications that tried to tackle the task of managing a single project (as opposed to Source Forge which is larger in scope). It was designed for smaller teams and setup is not that difficult, but more difficult than a nice simple PHP+mysql setup like Mantis.
Search actually seemed a bit slow & it doesn't provide an easy way to narrow down a tickets based on specific ticket field value (for example "Customer".
You can create custom searches via Trac Reports - which also look like they involve writing SQL statements. This is probably a nonstarter b/c it seems we would want a simple interface to help us narrow down tickets without having to resort to SQL - also I think these Reports are project wide so users would not be able to create their own custom searches.
You can add custom fields to Trac tickets by editing the trac.ini file but it looks like to get things to work smoothly you may quickly fall into writing python + SQL statements. It appears you can customize the interface it pretty heavily using "Genshi" which I take is kind of a mix of python and xml (yikes?). The fact that there is no simple web interface to add and customize the fields is a big negative.
Maybe there is a plugin for Trac that fills these gaps, but most of the howto pages on Trac's wiki end up with some kind of howto on writing Trac SQL statements. This leads me to think it is not the solution we are looking for.
I would prefer redmine (larger scope) or mantis (just a bug tracker) over Trac. Trac was fantastic a few years ago, but many other projects have started to play ball better in this space.