Just arrived in ORD (I believe T5) coming in from LHR. I'm on ESTA, so just took the regular ESTA line and told the CBP officer I wanted to do the interview for GE Enrollment on Arrival. He then directed me to the office which is on the left side right after passing immigration.
Very friendly lady then asked me a couple of basic questions, took a picture and fingerprints and all was done, took maybe 2 minutes.