I had a bad feeling about watching this (according to this awful rating) but I also thought this could be one of these few cases of raging viewers down-voting a good movie because of very personal reasons. And this seems to be the case.
To be clear: My voting of 8/10 is based ONLY on the premise of viewing this as a total work of fiction loosely based on some facts. I understand that watching this movie as a person who was directly and personally affected by 9/11 probably makes said person quite angry. This movie could easily been seen as cheap propaganda, but i guess this was not really the intention.
Long story short: This movie is very enjoyable if you like stage-play-like films where dialog is the main ingredient. Basically there is just a handful actors in one cheap location. But who cares if said dialog manages to keep you listening for the whole time? If you're open minded and like thought provoking films (even if they may be just propaganda): Watch it! You won't regret it.
BTW I seldom review movies, only if i feel that a movie deserves it.