The interviewer seemed genuinely interested in my research. He had read the master's thesis I submitted with my application and the bulk of the interview was very collegial. He asked why I wanted to pursue a PhD, what interests me about the subject area, and why I chose to apply to their school. It lasted about 25-30 minutes. There was plenty of time at the end for my questions as well. I think it went very well and was definitely a little boost after my Wharton rejection.