Pros
Great benefits, excellent university, good opportunities to advance your career in academia, and challenging, high-quality research environment. You are surrounded by some of the top minds doing interesting work on a truly endless range of subjects. A great place to be if you like to stay up-to-date on academic trends - my experience with researchers here is that they work very hard to get ahead. Evanston is a nice city, and you can enjoy some good outdoor spaces during breaks. My experience is that people in general are friendly and helpful in cross-departmental interactions. Also, there are good opportunities to move within the university after 1 year of service.
Cons
Low pay, mid- to upper-level management are not always competent, slow-moving administrative processes (like all universities, or so I've heard), high expectations with little reward or incentive. Because it is a Research 1 institution, the pace can be high-pressure at times. In my experience, staff are often not always recognized often for their good work. Negotiations about employment have been less than stellar, and some HR processes involve lists of seemingly endless forms for small changes (probably not unique to NU!) The location is nice, but you are slightly out of Evanston, so it's not always convenient if you don't drive.