I've seen the future. And it is entering into the U.S. with the Mobile Passport app. They recently added SFO, and it worked flawlessly.
It's the official app, no weird tricks here.
First, you snap a selfie and put in your passport details. That gets stored in the app for future use.
On a flight, you can enter your customs info, no need to use any paper customs forms. Then when you land and get data, you just submit it. Done.
But the best part is going through the passport control line, if you don't already have something like Global Entry ($100 application fee for 5 years). You zoom right past everyone to a dedicated line for mobile passports and voila. Using the Mobile Passport is totally free, unlike other fast track programs.
Highly recommended if your airport is supported. If not, keep checking to see if they add support for it in the future.
MOBILE PASSPORT service is currently available at Atlanta Hartsfield-Jackson International Airport (ATL), Miami International Airport (MIA), Seattle–Tacoma International Airport (SEA), Chicago O’Hare International Airport (ORD) and San Francisco International Airport (SFO), with more airport locations to follow.