I've had my passport since November 2010, when I had to do a rush-renewal in Philadelphia ahead of an unforeseen trip. It's technically good until late 2020, so I planned on renewing my passport after electing a new president (fingers crossed).
I had forgotten that many countries won't let you enter if your passport isn't valid for at least 6 more months. I'm also up to extend my residence permit later this year. My roommate David warned me that the Ausländerbehörde may want my passport to be valid for longer when issuing a new permit. How much time left they want you to have isn't clear, but all these factors combined made me decide to go ahead and get the new passport.
I thought it might be useful for other people if I write down my experience renewing my passport, since this seems to be a pretty common topic in the Americans in Berlin Facebook group.