I travelled couple of times using AP. Its simple. Make sure you carry I-485 receipt notice, AP and EAD. Keep copies as well. At the POE, give the officer your Passport, AP, I-94 and I-485 receipt notce. You will be taken to a separate room. Officer checks the database and stamps your passport and I-94. In my experience at Newark both times the waiting time at the room was about an hour and a half before my name was called. Officer called me and asked my SSN/sponsoring company and gave the papers/passport. Second time the same AP which was used in my previous trip was stamped.
