My last team switched from Subversion/Rietveld to Mercurial/Review Board about a year ago, and I have to say that I much prefer the latter.

Even leaving aside the advantages of a DVCS such as Mercurial, I think that Review Board has a cleaner, more responsive UI. The "interdiff" feature (i.e., show diffs between changesets in this review request, not against the trunk) is pretty great, too.

Also, as a fairly mainstream Django app, it's not dependent on the App Engine runtime, which lets you run it on any standard Python/WSGI-capable web server on any recent version of Python. (App Engine only supports 2.5.)

If you're using Mercurial or Git, I'd highly recommend you check out Review Board; the initial setup is easy if you've deployed any WSGI apps before, and the web API makes it pretty easy to tie into your ticketing/CI/IRC bot/etc.