I was referred to an analyst position by a current Mercer employee, and interviewed with a total of 12 people over a period of 5 months. This included initial phone screenings, several phone interviews and two sets of onsite meetings.
Most of the questions were standard (why Mercer, what do you think you bring to the role, do you have experience in this area etc), and each interview was an hour exactly. There was a large emphasis on having me ask them questions of Mercer, and in some interviews they barely asked me any questions at all and just talked about Mercer. The employees I met were mostly smart, professional and friendly. There were a couple of jerks (one guy showed up without my resume and was incredibly rude throughout the interview) but I think most companies have a few of those!
The HR department were very responsive throughout, and I got the impression that they were covering for an unresponsive manager. I've been waiting on a promised phone call for some time, so I'm assuming that I won't be offered the role at this stage.