I've used Trac as an issue tracker, and I've liked it well enough as that. I somewhat prefer Roundup for more involved issue tracking, but I think Trac has a more accessible UI for public projects. But I haven't felt any real motivation to use any of its other features (or Roundup Subversion integration for that matter). I don't need Yet Another Wiki (and worse Yet Another Wiki Markup). That's not how I want to keep my documentation. The Subversion integration might be good, but I never remember to link them up. They are too far away from each other, IMHO; web and svn client are very different modes for me.