Supernatural, from a technical perspective, is an amazing show. It has a complex storyline and characters, realistic special effects, and an incredibly accurate view into the human soul.
Now, talking about morality, you need to use your own judgement as to whether or not you can handle the show. Some adults would need to leave the room at violence a twelve-year-old would barely flinch at. Most of the gory scenes are censored, the most disturbing one I recall seeing on camera being a guy coughing up razor blades.
As for sexual content there are a few vanilla sex scenes with plenty warning to skip through. One of the characters, Dean, who alludes to sexual content the most, does it to hide how much he's effected by emotional trauma (one example being when he knew he was going to die soon he engaged in more questionable activities.
The main characters have fairly good core ethics and any religious content is clearly separated from real religions (like their portrayal of angels, demons, and Satan himself)