The technical interviewer I spoke was a pretty conceited and cold person, for whom seemingly you either know their open source library (Toothpick) and you're in or you don't and you're out. I could see that the moment I said that I was unfamiliar with it, the interview was gone. They couldn't resist adding "you really should check it out, it's great" in a patronising way. After that the person kept asking me questions on the topic I had already said I know very little about, seemingly just to kill time and watch me struggle. They really weren't interested in me as a person or what I did know or have achieved.

Great interviewing experience. Advanced directly to onsite through fast tracking process. Met with hiring manager (past projects and deep dive into Android knowledge), Android engineers (system design question on whiteboard), lunch with Product Manager (behavioral and culture fit questions), Engineering Manager (pair programming coding challenge on laptop), and finally with Director (more culture fit and questions about company). All people involved in the interview were really pleasant and knowledgeable and I felt valued throughout the process. Received offer the next day and HR was really helpful with allowing me time to conclude my other interviews before letting them know my decision. Judging from the interview process and the people I met Groupon seems a great company to work for.