I wasn't too sure I wanted to see this movie. I basically just watched it because it got great reviews, and because I like Casey Affleck.
This is a great movie. The storyline, the characters...and the ending completely shocked me.
Violence is definitely an issue. A lot of violence is shown, and violence toward children is implied. Language is a big issue, too. Plenty of f-words, and many, many more.
I think it was definitely appropriately rated R. Obviously, it's not for kids, but I'd highly recommend it to anyone over the age of 16.