When I saw the preview for United 93, I thought it looked cheesy and I was afraid it was going to resemble a made-for-TV reenactment. But when I heard some positive reviews, and saw that it had a 91% on rottentomatoes, I wanted to see it.
With such a sensitive subject, it would be very easy to go wrong - but Paul Greengrass did a great job of presenting a realistic view of what did and might have happened.
Leonard Maltin also liked it:

As someone who repeatedly chides the studios for making formulaic fluff, I can't condemn anyone for tackling one of the most significant true-life stories of our time. The film is impeccably well-made, executed with taste and restraint; it certainly pays tribute to the passengers of that fateful flight who took heroic action and changed the course of history.