The interview process was smooth. I received an invitation to interview with the Principal Engineer on a Wednesday and had my first round with her (a technical phone screen for about 1.5hrs) the following day. It was unusually long, but during that call, I learned a lot about their culture and how awesome it would be to work with them.
I had a 30-minute phone interview with HR the next day (Friday), then another with one of the co-founders (about 45 minutes) the following Monday. My last round was a 1.5-hour face-to-face technical interview with the Principal Engineer.