This was my fourth time running the Chicago marathon. I can't stay away! The energy is hard to beat. I ran the Boston Marathon last year and find that the energy surrounding the Chicago marathon is pretty comparable.
Some pros/cons:

Pros:
CROWD SUPPORT. ONE MILLION screaming fans. Literally. The energy is fantastic.
Volunteers: They're awesome. They're handing out water more often than you probably need it and despite the crowds, there's never a line.
Flat/fast: It's such a flat easy course. Perfect for beginners or those looking to PR.
Expertly run: corral placements, gear check, pacers, buses bringing runners to/from the expo. You can tell these people have been at it a while and things go off very smoothly.
Scenic: It's a great neighborhood tour of the city I've grown up in.

Cons:

Expensive.

Crowds. Even with the corral placements by time, expect to be doing some weaving and bumping elbows well into the race.

Swag. The t-shirts are almost always pretty ugly. If it's a big race for you, expect to pay extra for a shirt you'll want to wear. The medals are usually pretty great though.

This race was on my birthday year and I figured that it was a fun way to start my day. It started and finished in a mall parking lot. This was great in a way because it made parking extremely easy. It was still pretty chilly at this time of year so I was able to get my race packet, bring it to the car and stay in the car until pretty close to the start time, staying warm. The race includes a 5k and a 10k and the 10k was just two loops of the 5k course. You basically run around the mall and a business park and it's not terribly scenic. It's a touch hilly for the Chicago hilly but still pretty flat by any other standards. The t-shirt was ring-spun cotton and my daughter wears it like crazy because it's so soft. There was an after party and pretty basic food afterward. The race is pretty small and I was able to get an age group medal despite having a less than stellar day.