Well I've been to Mount Sinai a lot of times.... not to mention i was born there..
the staff is generally very nice and accomodating as long as you are calm and collected with them.. everytime i've had to go in for an emergency i was attened to within 20min so the wait wasn't bad at all. And they've been at random times of day from when it was basically empty to when it was packed.
however, i had to bring a friend in for horrible abdominal pains and we had to sit there for 5 hours (2am-7am) waiting for another doctor to come in which wasn't all that great, but the staff was very accomodating.
on the downside the second cup by the emergency room in the building doesn't open will 7:30 or 8am..