However, the key thing we do is actually make the 20% be a full two-week sprint, it just happens every 5th sprint. That way, you can have the time to do some planning, suggesting, and documentation before setting out to build something and you get the full amount of time you needed to build it. We purposefully do try to keep features limited to something shippable during that time period, though some folks will spread it out over a few for bigger efforts. It's pretty great and a key part of our culture that I'm happy we've been able to keep. It's also produced some of the best features Kiva has (as well as served as a great time for engineers that like to tidy things to clean up the codebase).
http://pages.kiva.org/buildkivablog/2011/02/10/kiva-engineer...