I did my interview on arrival today at PHL. There was a dedicated lane with six of us in line. You cannot use it with mobile passport so I had to get out of line to get a paper customs form. The agent collected everyone's passports to take them to the office then sent people to collect their bags. Since I didn't have bags I followed him to the office and interviewed first. The agent didn't ask me any questions, she just took my photo and fingerprints and sent me along. The entire thing including waiting for someone to collect our passports was 10 minutes and still much shorter than the regular passport control line. She said I'd get an email with my known traveler number when approved.