Flynn seems to have stalled, unfortunately. It was a pretty straightforward setup, and I got apps running on there rather quickly. There's still no real support for letsencrypt, which is kind of a killer.

Dokku has no support for clustering, but it's amazingly simple to set up and start deploying apps. (Digital Ocean has a pre-built and up-to-date image ready to go.) I like it for the heroku-style buildpack deployment, although it also has dockerfile-based deployment. Great little system for low-risk projects.