Not really worth the effort
As a lover of Urban Fantasy, I had to check out Supernatural. My conclusions were...well.. On the good side: I love how the series tries to re-adapt concepts to a more modern day setting, with things like the whole "carnal vessel" thing for both angels and demons, the Four Horsemen of the Apocalypse driving cars instead of Horses, or the Urban Legend creatures adapting to modern day technology. It's all very creative and interesting, and by far the best thing the show has: The creative mythos. It features a skilled cast, which helps to carry the seemingly ridiculous things that happen. Also features a great soundtrack and score. On the bad side, I'd say it's one of the most dramatically repetitive shows I've ever seen. Every single season features Sam and Dean finding some seriously asinine reason to be mad with each other in the most ridiculous situations. Sometimes (in the earlier seasons) it could be interesting, but it was very hit-or-miss, and sometimes it'd just come off as shoehorned drama for drama's sake. Another criticism I take with this show is how it seems to have a passion for Anti-Climax. Not one single climax of any of the seasons was satisfactory, the show keeps building angels and demons as these massively powerful entities, but it's one of the most egregious cases of Informed Ability in the history of fiction. Ex: Lucifer and Gabriel are supposedly the top dogs of Angellic hierarchy, and their battle is a Single-Stroke Battle that lasts five seconds. Three of which are dialogue and stares. They don't have the budget to display massive power clashes, but surely they can do better. Angels fight each other with knives, make good knife fights instead of ridiculous anti-climatic stabbing. The show's Anyone Can Die vibe also works against itself. Most of the interesting characters (Gabriel, Uriel, Raphael, Michael, Alistair and the Yellow-Eyed Demon to mention a few) are killed randomly (and literally not one of them gets a proper conclusion), wasting their potential, which is aggravated by Sam and Dean's shoehorned melodrama. It also has a bad track record with them. Most of the villains are flat and generic (I am looking at you Lilith). In conclusion: It's a show with great potential conceptually-wise, wasted to all Hell by terrible writing.
A great series
Supernatural is a great show. From the awesome cast to the cool mythology to the great self referential humor, this is an amazing show. As of writing this review, we are nearing the end of season 8. Supernatural's best seasons are 2, 4, and five. A must watch for any science fiction, horror, and action fans. My favorite television series must live on. By the way, the demon Crowley is an awesome villain.