Granted, I've only run one marathon, but I doubt that you'll find a much flatter course than Chicago. Everyone had prepared me for the hill at the end (Mt. Roosevelt), so I knew it was coming, and it did not feel as bad as I anticipated.
Weather was gorgeous, albeit a bit too warm. What are you going to do? You never know, with weather (in Chicago). The warmer weather brought out the spectators, and there wasn't a quiet spot on the course. Again, I couldn't imagine running for this long without all the cheers and funny signs from spectators.
My one gripe was my pace group. They were way off. I knew that my marathon pace would be faster than my long run training pace, but we started off and it just seemed WAY too fast. I kept it up, though, until mile 9. My husband confirmed: we were going 10-15 seconds faster per mile that we were supposed to.
Starting out too fast (aka, breaking the cardinal rule of running) really was detrimental for me. I should have listened to my body and left the pacer right away, instead of waiting until mile 9. This complaint is more for the pacers (sorry Nike) than for the organizers of the Chicago marathon.
If I *ever* run a marathon again, I think it would be Chicago for the flat course, huge spectator support, and just knowing what to expect.