My wife did EoA at ORD on Monday, Dec 30 and it was an easy process. Here's a quick description of what she/we went through.
My wife, myself and our infant daughter applied in September (first time applicants). My wife received her conditional approval on Dec 24 while we were traveling outside the US (me and my daughter still waiting for CA). With our planned flight back to ORD, arriving around 3.30 PM on Monday Dec 30 and 3hr to catch our connecting flight home, we thought the timing was perfect for EoA. Our flight was on time so we got to immigration line a little before 4 PM (on a Monday). We tried to find but didn't see any sign for enrollment on arrival. We asked someone who worked there and were told we should go and see an officer and get admitted to the US first then he/she will tell us what to do next. We did exactly that, standing in line, seeing an officer at a regular booth (not a GE booth/area), getting processed and admitted together and then he told us to go to another booth around the corner to complete the EoA process. I asked him if we could go together even though only my wife was going to do the EoA, he said yes. We walked there together and there were a couple of people in front of us. My wife stood in line and I took my daughter to the baggage claim area. Not long after I finished picking up our luggage, I could see my wife packing up her stuff and walking toward us. It was that quick. She said it took probably 5 minutes, including photos and finger prints. She didn't have to show the CA letter, they just looked it up on the system. They only asked a couple of questions regarding her travel history (e.g What did you go to XYZ country for?). By the time she joined us at the baggage claim area, which is a just a short walk away, an update came via email.