I think the Go project uses Mercurial but does code reviews using Rietveld. They have some python code which helps in sending patches for codereview to Rietveld.

I haven't used Review Board, but I have however used Rietveld and I don't have any complaints about the UI. It also provides the "interdiff" feature, just choose the patchsets you want compared for a specific file, when you go to the diff page.