Hey guys, I interviewed at Northwestern yesterday (2/18) and I thought the whole process wasn't bad at all. I was pretty freaked out beforehand about interviewing in front of a panel, but it really felt more like an informal discussion than being grilled by the interviewers. I think there were about 20 of us there, altogether, and everyone was scheduled for their panel interview either at 9am or 10:30am (mine was at 10:30). Most of the panels consisted of 4 interviewees and 3 interviewers (2 MD's and a 4th year). Either before or after your interview you'll meet for about 15 minutes with either Dean Brown or one of her assistant deans. This is a very relaxed meeting that's really just to verify your amcas info.
So some of the people I talked to liked the panel setup, but the majority didn't. It's tough to fully express your thoughts on an issue when there are 3 others there who also have to talk, but just try to be concise and prepared. They'll probably ask you "why medicine" and "why Northwestern" so you should have a good idea how you'll answer those questions, but also be prepared for discussing an ethical issue in medicine. One of the groups had to collaborate on a question that went something like: "If you were given x number of dollar to spend on some medically-related venture, how would you spend it."
Be prepared for a long day and try to get plenty of rest the night before. You're there at 7:30am and don't leave until 3:30pm. There were at least a couple of us who were having a hard time keeping our eyes open during the financial aid session at the end of the day. Dress for cold weather (though it wasn't too bad when I was there) and wear comfortable shoes.
Overall, I had a fantastic time at Northwestern and I'd love to go to school there. You'll find that the students are all incredibly enthusiastic about being there and I met a nubmer of students who turned down great schools to go there (Hopkins, UPenn, Cornell, etc.).
Good luck with your interviews, and PM me if you want any more info. And make sure that you act as CONFIDENT as possible during the panel interview (Dean Brown mentioned this in her intro). Act like you really belong there and you'll be fine.