I met United at a career fair at my school. I interviewed at their HQ with 3 people. The first interview I had that day was mainly about my background, gathering why I had an interest in the company, etc. The second interview was more about how my skills, experience and interests were directly applicable to the job. The third interview was a case study. The toughest part of the interview is that it is hard to gauge whether or not you said something that they wanted to hear; the first interview was a nice way of easing into things and more casual, but for the second and third they were very neutral in giving you feedback from your answers. All in all, I'd say it is neither the easiest nor toughest interview to overcome; just come with some proper preparation (although, I had never actually "prepared" for a case study question before, and did fine by winging it and applying my technical knowledge).