I had three total sessions. The first two were technical (problem solving) which required whiteboarding. They were conducted with pairs of interviewers and lasted about an hour each. The difficulty was reasonable and I made a few mistakes but ultimately did well. The third session was with the director of engineering of the team and wasn't really an interview. He just told me that I essentially got the job and was done. Overall pretty easy compared to Google, Facebook, etc. But keep in mind that this was early 2014. Now, the bar is much higher and the process is much more formal. Don't expect to just wing it; you should practice and take the interviews seriously now.