I can tell you it is ten time worse for relatives than if you are there. I knew I was ok, but they did not and without mobile phones or landlines working - there was no way of letting them know either. Unfortunately, a friend had called my wife to ask - and my wife was not aware of the disaster at the time which made matters worse. It took just over an hour to get a connection through my laptop via a faxline which still worked. At least managed to speak to my parents from there as well as again, anyone trying to call my mobile would just get a "dead" tone. It did not help of course that I had sent pictures from my view earlier, so they knew exactly where I was in relation to the towers.
The one thing you will never forget is the smell.
I did not leave my building until much later that day, remember that nobody knows or can tell you anything so it seemed to make more sense to me to stay there. I learned more from the TV than from being there.
From a personal point, the fact that you are in trouble abroad with no means of getting out of it is not pleasant, with all your family on the other side of the world. Rather lonely position to be in (pure selfish observation of course as the people around were of course much worse affected as I did not lose anyone).
Other than what you all have seen on the TV, the main difference was that the TV "live" feed was about 2 minutes slow, meaning that when the towers collapsed - they were still standing on the live images on the TV).
NY just continued with most shops being open on 5th Ave. Trying to get a flight back was rather a challenge and could not be organised from within the US as the airlines seemed to have shut down. My original scheduled flight was on September the 12th which was obviously cancelled.
The way I managed it was calling KLM in Amsterdam, book a flight from NY to Canada, then book a flight from there to Shannon, from Shannon to Dublin, from Dublin to London. Check-in and security times were rather long as expected, so the entire trip back was about 3 days.
Did the memorial tour a couple of months back when I was back in NY, they have organised that very well. My wife and children did the firefighter tour about 8 years ago, which did upset them at the time as it makes it more "real".
Very sad event.