I watched this movie with my 14 and 15-yr-olds, mainly because I wanted them to fully understood what happend on 9/11. They were only in 4th and 5th grade when 9/11 happened, and while they were aware of the significance of that day, they were not really aware of all the details and they weren't fully aware of all the bravery and heroism that shone through on that day.
Yes, it is a violent and sombering movie, but it is also a very important part of our recent history that our children should be fully aware of. Teenagers who were only elementary school age at the time of 9/11 are now able to comprehend more fully what happened on 9/11.
Also, while this movie only portrays a certain view of that day, it does a great job doing it. No movie can represent everyone that was impacted by that day. Each movie made helps us all piece together all that was of that terrible day. I would make the same comment about World Trade Center - while it focuses on only a few people, it does a great job showing us that perspective.