Pros
Good benefits, fair treatment of post docs. Northwestern's name recognition is extraordinary-- everyone in the Chicago area seems to respect Northwestern. In my particular field, NU's reputation is strong, and I've found that this can open doors. After one year many postdocs qualify for retirement benefits. Health, vision, and dental benefits also seem pretty reasonable. Not great, but the situation could be much worse for postdocs. This will all vary by your field, department, and lab, but my boss is reasonable. I'm pushed to complete my work, but my boss understands balance. Depending on which department you're, you might be able to take advantage of excellent job-hunting resources offered to postdocs. Northwestern does a pretty good job of preparing it's post docs for the next step. You wouldn't want to work here as a postdoc forever, but for 1-4 years, you could do a lot worse. With any luck, you'll leave with a great set of skills and contacts, as well as the Northwestern name on your CV. There are plenty of cons (see below), but if you're looking for a job for a few years, you don' t have lofty academic goals, and you still like ramen noodles, Northwestern is a decent place to be.
Cons
These cons are specific to my experience as a postdoc in one of the physical sciences. My work is almost 100% research, and at NU it's impossible to build any momentum. As soon as my experiments start working, I seem to have to stop work to deal with: (1) bureaucratic hurdles or (2) instruments that are broken or too heavily booked. Work that can take days at other universities can take weeks or months here. The bureaucratic hurdles mean that ordering lab supplies is a nightmare-- it can take a week (or more) to place an order, and after the package is delivered to campus it can take days to receive it. As for instrument time, if you use shared equipment you might find that you need to book time more than one week in advance. It can take weeks to shift gears if your goals change suddenly. I find myself plodding along when I'm dying to sprint. If you're a "slow and steady" type, or you relish filling out forms, this could be the place for you! There's lots of good stuff about Northwestern (see above), but I find that the expectations here are quite high, and the resources available make reaching those expectations almost impossible. These cons are particular my particular physical science.