Tip: If you are going to Chicago, see if Midway (as opposed to O'Hare) is an option. I went through Midway for the first time this week on AirTran because the ticket was 1/3 of NWA or United to O'Hare. What I found was a nice airplane with XM radio, no delays, and a train ride to and from the Loop that was about 30 minutes, as opposed to 45 minutes to and from O'Hare.
In sum, as far as I can tell, all O'Hare offers over Midway is (a) more congestion, (b) more delays, (c) more money, and (d) longer commute times. Nice selling points, don't you think?