Of the 4 ORD machines I tried on a recent early morning arrival, only one was working. No one around to offer assistance. The area had no border patrol. I was contemplating just walking though but knew there was no upside to this. Other passengers continued on to the next section.
I finally got one of the four in the unmanned section to work after I had tried the other three and waited for another, successful, passenger.
When the machines work, it is brilliant.
When the machine does not work, just walk to the front of the line and show your sticker. I actually traveled with daughter who does not have one and went to front of line with my sticker and her and no problem