I'm a candidate that will be interviewing at NW later in the season. I don't see any other posts so I will help with what little I know.
I've heard the following:
1. University of Illinois at Chicago (UIC) and Northwestern are the two strongest programs in Chicago. I haven't heard whether either of the two programs is clearly better than the other. University of Chicago is in a "transition" state as they have lost certain key faculty members to other programs.
2. NW residents work longer hours than UIC residents.
3. I've only heard positive comments in terms of whether or not the residents are happy.