Watched it as it happened in Operations at work, thinking how we'd end up going somewhere sandy. It's my birthday too, so not something I can ever forget.
I remember thinking quite cruelly that it was a wake-up call to the Americans. I'd spent years doing anti-terrorist stuff, knowing that "patriots" in the US were helping fund the IRA. years of checking my car for devices, years of driving to work hiding my uniform. Remember the nail bombs? The barracks bombings? Hyde Park? Big events in Britain, small events in the World.
But 9/11 was an horrific sight. My brother-in-law was evacuated from an hotel nearby and ran with thousands of others to Central Park. It was a long wait to hear that he was ok.
Then we had the pleasure! of endless trips to the Gulf, the fear of flying into Basrah at night, wondering if our anti-missile defences would work properly. Listening to the base getting rocketed around us. Watching the firefights below in Al Amarah, Najaf, Baghdad, and Basrah itself. Scrambling to assist in a TIC (Troops In Contact). Collapsing in horror when our friends of many years died in the Nimrod crash.
So 9/11 was a continuation of crap, but in an unimaginable scale. And as someone said above, the effects will go on for many years to come.
It also taught me how much the Americans regard their Armed Forces, and the indifference the majority of British civilians had to us. Our visit to New York and the site was full of support, thanks, praise for what were doing, and it was simple and honest. Nowadays Help For Heroes has changed that. So many UK families now have a link to guys who have headed east.
So that's part of my take on 9/11. Other views are available.