This is a show that really just depends on the person. If you have a week stomach, I wouldn't recommend it ( as there are beheadings, plenty of blood splatters, sometimes gruesome bodies, and the likes.) However, horror wise, its often toned down by several comical moments throughout the episode. I get frightened easily with ghost movies, but this show doesn't scare me at all.
Besides the violence there is an occasional sex scene, but often times they're not very graphic. There are quite a few references though, usually for a comical reason. But they aren't very crude, and could possibly go over younger teens heads.
There's controversy over how bad of role models the Winchesters are. True, the boys (especially Dean), have a very casual attitude towards sex, enjoy a beer, and dont feel very remorseful towards killing (though they kill things to save people). But personally, I think that's what makes them such great characters.They aren't almighty heroes. They seem real. And when it comes down to it, they do what they do to save lives. And the boys would do ANYTHING for each other, which offers up some very strong messages about the importance of family.
As for drinking and drugs, the Winchesters enjoy a beer about every episode, but are rarely shown getting drunk. Demon's blood is acted as a drug, but its not glorified and is shown to overall have gruesome effects.
To sum it all up, if you can stomach it, I highly recommend it.