Personally, I love this show. The acting is superb, and it's easy to fall into the character's personal lives and backstories. The positive messages far outweigh the negative ones. Of course, there's drinking, violence, the occasional sex scene, but it's competing with strong family bonds, close friendships, love and loss and hope. The first couple of seasons are truly splat-tastic- Season One brings back visions of old bars, one-lane highways, warbling, dark rock, smoke, guns, sleazy motels, blood, guts, gore, and some rather horrific exorcism scenes. The brother's attitudes towards women- especially Dean's -were almost degrading. 'Bitch' is an insult that tends to be tossed around a lot, as well as damn, hell, etc. Nothing too bad. The drinking, violence, and otherwise negative themes are the base for the first two seasons.
Around Season Four, things become less 'let's-go-kill-some-nasties-get-drunk-pick-up-a-chick-and-drive-off-the-next-morning' and more 'I-need-you-so-much-more-than-you-could-ever-realize'. There's heart-plucking death scenes, emotional bonds that are shattered painfully, betrayal, hatred, and circumstance.
In the end, I really do think it's about two brothers who just want to do good. They were forced into this because of an accident cruelly executed late at night, because of a father who really just wanted the best for them and who was so crazy with rage and grief that he ended up getting killed himself.
It's hilarious, sob-inducing, wild, cold, angry, elated. It's about being brave and standing up for what you believe in. It's finding forgiveness and light in the iciest, dankest places. It's about finding your family and weaving tight friendships that nothing will be able to unravel. It's about humanity, and how not even the Devil was born evil.
I think it's about the secrets of the stars.