There are plenty of places in chicago you can go to to see average stand-up. But Second City is one of the few that offers consistently funny shows by professionals. There's always a couple shows running, and they are very impressive. Well written, well acted, just funny. Some sketches are hit-or-miss, but that's comedy. They usually do some improv after the show, giving you a free second show.
Food and drinks are kind of expensive, but no more so than any other bar. The shows are about $17 each, which is quite reasonable considering the quality. Great place to take a friend visiting the city, or just to go with friends.
The seating is kind of cramped, because they squeeze a lot of people into a fairly small room, but that also means everyone is pretty close to the stage, which is a nice effect.
PROS: inspired comedy