That is really a tough decision. Personally, I would not try to go downtown. What with the travel time plus the possibility of delays, you really wouldn't have that much time in the city.

If you do want to get out of the airport, there is a Nick's Fishmarket in Rosemont (last time I checked; suggest that you verify) so you could take a cab over there.

But if I were you I would just hang out at ORD. The DL club and people are great, and IIRC there is a nationally known restaurant at the end of the AA terminal.

Don't get me wrong. Chicago is a wonderful town, and you should find a chance to spend at least a few days there sometime. But for a 5 hour layover, dealing with security and luggage, as well as transpotation, I would just chill out at ORD.