The Chicago Marathon was my first world major, in 2017, and I LOVED it. Everything about it was well done and I was continuously impressed and have recommended this race to all my friends.
The race expo is massive and a bit overwhelming, but if you come in with a plan it can be super fun. Bib pickup is a bit more intense than most races, but once you find your pickup location within the expo it's very easy to grab your bib, then make your way to the back of the expo to get your shirt and goodie bag. The vendors at the expo are top notch and the Nike branded gear is terrific. The year I ran the expo had buses running from downtown Chicago to the convention center, which was super convenient and easy.
I was nervous heading into race morning, as my friends and I had stayed north of downtown and had to take the subway to the start. It turned out to be incredibly easy; we got off at the stop that was closest to our corral and went through the assigned security gate. The process of checking bags and getting through security was fast and we had plenty of time to stretch and hang around the corral before we started.
The ONE downside to this race (which is probably true for any race in a large city) is that GPS is very wonky for the first mile or two as you go under bridges and around the tall buildings. My coach warned me of this ahead of time, so I set my watch to look at the elapsed time and ignore the distance until I got out of downtown. After that it was smooth sailing! The aid stations were well stocked and frequent, and the crowds were incredible!
The course is very flat and the only incline comes in the last 1/2 mile as you turn the corner and enter the park to get to the finish line. This marathon wasn't my first, but it was the most emotional! The skyline and the crowds at the finish line were surreal and it was an incredible experience.
The finish chute is long and it takes a while to get to bag check and then to the finisher party area. It was relatively quick to get to bag check; I recommend stashing a snack in your checked bag!
Overall I loved this race and can't wait to do it again in the future!