score:2
I have now traveled through O'Hare. The answer to the question turns out to be irrelevant, since all visitors are now directed to use the APC kiosks. So as far as I can tell, APC is mandatory at O'Hare.
Upvote:2
The main reason that you have to get in line the first time is that they take your finger prints. Every further visit, the machine can match them to your prints, and identify you.
First visit (or if you weren't finger printed), obviously it can't, so you will have to go in line and see an officer.
Upvote:6
In my experience, anyone entering under the VWP can use the APC machines (which saves filling in the paper form at the least) but if you have not entered the US before with your current passport (possibly using the same ESTA but have not been in the position to check this either way) then your paper slip from the APC machine will be marked and you will be sent to speak to an officer as if you had joined the non-APC line to start with.
At JFK you are sent to a dedicated queue for people who were flagged up at APC, this may or may not be the case at Chicago. This queue can, however, take just as long as the main queue - it may be minimally staffed and thus move very slowly despite its shorter length.