I have seen it before too... I don't know if it's because I flew to Greece on Sep 12th 2001 and missed the intense news coverage that followed, but I still find it hard to take in what happened that day. When I see the plane hitting, the people jumping to their deaths, the towers collapsing and just the sheer size of the whole thing... even the ash cloud which covered most of Manhatten shocks me.
And as for the Tsunami on Film" programme... I have never cried like I did when I watched that. Seeing the father and daughter looking through the photos of the corpses looking for their daughter/sister was one of the most horrific things I've seen.. their grief on finding her was unbearable