There's nothing like running through this great city with 40k of your closest friends. This is my second year running the Chicago Marathon and I was blown away again with how well a large-scale event can be executed.
BoA and all of the sponsors really make the experience special. They do a great job of getting participants all the information they need before, during, and after the race. The expo experience, while not extremely convenient to get to, is so much better than going to Navy Pier (as many other reviewers have noted).
Advice on raceday: get in your corral early. While well-managed, it does take some time to weave through the security checkpoints and corral gates. Bring a layer to ditch if it's chilly definitely use the facilities beforehand!
The course: this year a blue tangent line ran the majority of the course, showing the shortest possible route to run. While I was cognizant of the line, I used it mainly to set up for corners rather than elbowing people out of the way to stay on it. This is a flat, fast course that only suffered from a few wind tunnels heading south. The crowd support was much more consistent this year - it has been sparse in the past in the later miles. Great aid stations and volunteers all around. Do you best to give it your all up Roosevelt Mountain. Once you turn that corner it's less than 200 meters to glory!
Post race: the long walk down Columbus seems ridiculous but it does allow your legs to settled down and there's lots of opportunities to catch up with fellow runners for pictures, swag, and free beer (you don't even need your coupon)! I have not made it to the 27th Mile party either year so I can't comment on its execution.
My experience: This was my 3 marathon this year and 4th including Chicago last October. Each passing race I've gotten closer and closer to my BQ goal, falling minutes and seconds short every time. Finally, Chicago, 2014, I can say I have qualified for the Boston Marathon with a time of 3:02:06. I can barely put into words what this journey has been like but I'm overjoyed to say I conquered it at home! I will never forget the feeling of crossing the finishing line, knowing I had put my body and soul into this race.