I'm not sure how well adopted mongrel2 has been. There ain't many commits by Zed these days and I don't see blog posts about using it. I don't know why though.
I've been using mongrel2 pretty much since the beginning. I host my own websites with it. Although they are very low traffic I ran some performance tests using blitz.io and the results were impressive. Less than 1% conn errors with a 512MB linode and a simple Django app (no database).
