I've seen it. There are some scenes where he is drug taking - not shooting heroin or anything, taking tablets. There is also a scene where in a drug induced rage he wrecks the stage, there is some swearing, and one scene where he (cash) is putting up picutres of June in his house and his wife takes offence and he pushed her to the ground. It doesn't escalate in to a fight or anything though. There isn't anything too scary, but i personally wouldn't take children under 12, for the simple reason that they would get bored, and it's not aimed at kids.
I would thoroughly recommend the film. The acting by Joaquin Phoenix is fantastic, he has got the depressed, darker side of Cash off to a tee, and his singing is great. The on screen chemistry between him and Reece Witherspoon lights up the screen, the way he looks at her sometimes you would believe he (Phoenix) is really in love with her. He is very hot in it, my Dh told me to stop drooling!
My only complaint was the fact that the entire Tayside branch of the Johnny Cash fan club seemed to be sitting next to us, and clapped and sang most of the way through it.