Overall, worth watching at least once
The thing most likely to turn children off to this movie is probably its violence. It's essentially a thriller for kids. There is some weapon use and a few close-up blood scenes, but nothing grotesque that should make a preteen child too squeamish. There is a mildly uncomfortable scene in the movie where a man refuses to talk and is injected with a fatal dose of something, but that's probably the scene most likely to make younger children cry, and if you can handle that, you should be fine.
The scene most adults talk about in reference to the violence is the spectacular train wreck. In my opinion, there's nothing very wrong in the train wreck. It was not filmed to be scary, it was filmed to be dramatic, fantastic, and thrilling.
There are a few issues with language, but as long as your kid knows not to say any of the bad words, it will be fine. There's also a scene where someone briefly tries to sell them pot, but generally your child is too young to understand, or old enough to know it's wrong.
The movie has an overall positive message to let go of past hurts, move forward, and not hate something just because you don't understand it. The only bad role models in the movie are exaggerated, so it's very obvious that they're the bad guy, and there are plenty of good people in the film.
For families, I'll rate it an 8 out of 10, for some scary moments and mild violence.
For quality, I give it a 9.
What other families should know
Great role models
Too much violence
Too much swearing
Too much drinking/drugs/smoking