Clean, convenient and comfortable.
The Hilton at O'Hare is ideally situated if you have an early flight or are arriving very late and want to avoid the drive into Chicago. While the airport is only about 30 minutes from downtown Chicago, the drive could potentially take much longer if traffic is heavy. Thus it takes the worry out of the commute if you wake up and you're already there!
The rooms are reasonably large, and well-maintained. My room also had a large flat screen TV that had inputs to accepr HDMI, RCA or VGA from my laptop. A feature I'd like to see in all hotels! I even had a very comfy recliner to relax in. The bed was very confortable and the bathroom was spotless!
My only complaints were that the thermostat in the room did not go low enough (lowest I could set it was 65F/18C) which is a bit high for us Canadians used to lower nighttime temperatures. 
Also, the elevator noise was quite loud. While the front desk staff did ask me my preference of being close to or far from the elevators, and did also ask if 8 rooms away was far enough (which I answered would be fine), my room actually turned out to be 2 rooms from the elevator, and all night I could hear them going up and down (especially frequent starting at 4:00 am). You can also hear the occasional jet taking off, but it's pretty muted.
It is fairly expensive, but it's worth it for the convenience, Perhaps next time I'll really be 8 rooms from the elevators!