I just used it in Atlanta. Allowed me to skip the (longer) line of people lining up to scan passports in the ordinary kiosks. But then I had to stand for about five minutes in the post-kiosk line waiting to see the agent. Handed him the phone, he scanned it, asked me a few questions, no problem.

Note that there could be one significant downside to using the mobile app. If you are sent to secondary inspection, and the officer wants to search your phone, you can normally say "no" and just refuse to input your password.

But if you are using the mobile app, the secondary officer could conceivably ask "Did you use the mobile app, can I see your bar code?" Then you turn on the phone, enter your password, show him the bar code, and he keeps your phone and searches through everything.

My recommendation to anyone using the mobile app is show it to the primary inspection officer, but don't turn on your phone and input the password in secondary inspection for any reason. Alternatively, pull up your bar code before approaching the inspection area and save it as your lock screen. That way, when you hand over your phone, they don't have full access to it.