Yesterday me and my fiance returned to America (via San Francisco together). She entered on her K1 visa. The POE was at San Francisco International airport. The process was pretty easy!
At SFO, they split the line into two: one line for US citizens and another line for everybody else. I went with my fiance to the line for all the others. Just as we got to the front of the line the immigration officer goes on a break so they sent us to another booth/window. The immigration officer looked at Lan's passport, my passport, opened the K1 immigration envelope that was given to us by the US consulate in Vietnam. He reviewed all the papers, asked us when we intended to get married and reminded us to follow-up by filing AOS papers. He then approved it and that was it. VERY easy!