I watched United 93, because I had somewhere read that it is based on WTC related hijack. The movie was very gripping and very realistic. The sense of horror and unbelievability was evident on faces of many in the interval (or may be it was my state of mind). Was feeling sorry on one hand and was wondering how real this movie was. Later found out from net that it is based on real data except for may be few cinematic additions.

The movie is about that hijacked plane on the 9/11 which did not hit its target. Other 3 planes did. This plane was saved by the brave passengers on the board. The tensions/ confusions on the board, in the cock-pit, among those who were tracking the flights etc was filmed effectively. Without the interval, when I sighed, it could have been even worse.

More details about the movie here