We use Gerrit at SAY media to help us with the code review process. It has been working quite well for us. Lately, we configured the plugins so that hudson (well, soon replaced by Jenkins) also gives his 2 cents on each proposed change (That is, it votes -1 if the test suite fails).
Gerrit has come a long way from a documentation and installation perspective, so it's much easier to install and maintain that it was a year ago, kudos to the Gerrit team at Google. But now, if you want you can even skip this step by using Assembla's integration:post and video demonstrating this capabilities in details. Isn't that great to be a developer in the year 2011?