Skimming the other reviews, it's pretty obvious this is a bucket-list level event. If you haven't run the BoA Chicago Marathon. Stop reading this review and start planning when you'll do it. While it doesn't have as rich a heritage as the Boston Marathon or the international appeal of the City That Never Sleeps, the BoA Chicago Marathon is a world class event in a world class city.
Compared to the complicated start logistics of Boston and NYC, Chicago's loop course makes things a bit more manageable, taking some of the stress out of race day.
If you're looking to run fast, Chicago's flat course is as good a place as any if you're well conditioned to pavement pounding. An elaborate corral system also helps space athletes by ability. That said, if you're looking to PR, head to the front of your corral to avoid frustration.
The second half of the course can be brutal as there tend to be fewer spectators and a bit more exposure to the sun if it is hot. That said, I love running through Pilsen. Chinatown would be fun if it weren't at mile 21.