OP here
Yep, I ended up doing mine in JFK also. I had an appointment but they were practically pulling people off the street. Very nice guys, super efficient and happy to take you if you don't have an appointment. I chatted with the guy for a while, he said the AMEX promotion had a huge impact on the number of people signing up. He told me the program has been around 2-3 years but few people even knew about it before.
I'm based in SFO and there is practically NO availability through the end of December. What a difference! Plus, JFK has much longer hours, and is open Saturday and Sunday.
Anyway, my passport worked at the kiosk, as promised, 24 hours later (I was on a MR to Madrid so I got a chance to actually check the 24 hour claim). Very easy, and I'm very happy.