Had vacation in Mexico and re-entered the states yesterday through Houston using AP for the 1st time.
We were taken to a separate room and waited there when an officer was reviwed our documents. About ten or twenty minutes later, we were told that we were free to go. No questions were asked at all during this process. When we received back our documents from the officer, we found that he gave back to us all 3 copies of AP. As a result of this, we asked him if he is supposed to take one copy away. He said it's unnecessary. Therefore, we left.
The whole process was smooth. I think this experience might be of your interest.