Like Myate, my experience with Chicago O'Hare was unhappy.
Arriving in Chicago from Dublin, the flight was on time and there was sufficient time to, like 26cb said, collect my bag and take it through security where all luggage was opened and examined in the owner's presence before re-checking my bag for the onward flight.
The problem arose on the return leg of the journey when the internal flight back TO Chicago was delayed (still not sure exactly why.) As the transfer time was tight, the one hour delay meant that there was little hope of making the flight to Dublin. After a frantic race through the airport corridors, I arrived in time to see the plane being pushed back from the gate.
American Airlines, who operated all the flights involved, were unhelpful and I was pretty much left to my own devices alone in Chicago in a heatwave before I got the flight to Dublin 24 hours later.
The airport itself was ok. You'll probably have to change from one terminal to another. You catch a train to do it. Just make sure there's enough transfer time included in your itinerary to allow for possible delays.