My wife and I had our passports stamped after a 6 hour ordeal that started at 8:30 a.m. and ended at 2:30 p.m. Here are some tips based on what we experienced,
1. For an 8:40 appointment, get there before 8 if at all possible. They were not checking appointment timings, so people with 9/10 a.m. appointments were ahead of us in the line. All they care is that you have an appointment for that day.
I had my interview back in November, at the Buffalo (NY) DO. It was so quick we were in and out of the office within 10- 15 mins. We had a very nice woman officer.
She asked us to swear the oath, she asked us 3 questions and wanted to see our utility bills.
Once she had photocopied them, she came back in the room and said my GC had been approved. She said i should recieve it within 2 weeks, which i did.
Hope everyone's GC interview could be so quick and easy.