I’ve seen every single episode from teh very start, and own most of the shows on DVD. it’s no different than any other tv show; if you’re old enough and spiritually mature enough to understand it’s nothing but a tv show, there’s no harm in it.
It does deal with occult practices, but it also asserts the existance of God, angels, the afterlife…some times in a wonderfully positive way, sometimes not. For example, we have many episodes that show the CLEAR dangers of messing with magic and otherwordly forces. Demons are always shown as evil, soul wrenching beings that belong in hell. There’s an episode early on where Dean is faced with the existance of God, and forced to accept that he’s real.
Which is why one should be spiritually grounded. It can also be somewhat crass at times, but it’s not nearly as bad as a lot of other mainstream tv.
Watch it, enjoy it if it suits your fancy. I would definately not allow younger children to watch it though