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.