If you haven't seen this movie, then you probably shouldn't read any more of this because the whole thing is one giant SPOILER.
Having gotten that out of the way, let me say that I liked A History of Violence quite a bit. I was reminded of a couple hundred westerns I saw as a kid, the ones where the gunfighter is trying to give up his life of violence and start over. This movie's not quite in the Shane mold, but that should give you an idea. Someone always comes along and screws things up.
Viggo Mortensen plays Tom Stalls, who's not really the guy he appears to be. When a couple of murderous drifters come into his diner with intent to kill, Tom's hidden depths are revealed to the town and to his family, none of whom had any idea about his past. Will Tom be able to hang onto his present, or will he get pulled back into the old ways? Will his family be able to forgive him? Has his son inherited Tom's violent nature (and his reaction times, along with his fighting skills), and if he has, can he control himself? Are there situations in which violence is the only answer? Most of these questions aren't resolved, but they're there for you to think about.
The movie has an almost leisurely pace at times, and the violence, when it occurs, is over quickly. Things happen fast, and then they're done. This morning I was talking to a woman who recently survived a terrible traffic accident. She saw Jeep Cherokee swerve off the road and into a ditch. She said, "I thought it would flip over and that someone would be hurt or killed, but I didn't think it would be me. The next thing I knew, the Jeep was back on the road and on the hood of my car and bam! that was all." The movie reminded me of that.
There's a great scene near the end with William Hurt, who was nominated for an Academy Award for his brief appearance. He deserved the nomination. It's the best performance I've seen from him in years. He's scary and hilarious at the same time.