Before starting, I should note that I am a fan of the TV series, but I will review the anime as a standalone piece. I will put myself in the shoes of someone, who has never heard of Supernatural and evaluate its merits and faults from there.
Story: 7/10
The story is simple and easy to follow. It is about Sam and Dean, two brothers who hunt demons and evil spirits. Most of the episodes are following the “monster of the week” formula… but there is a general plotline that advances slowly throughout the series. There is also a main villain and the Winchester brothers have a personal score to settle with him. Unfortunately, there are a few filler episodes with weak story and unoriginal enemies.
Characters: 9/10
The personalities and the interaction between the two brothers makes the difference between Supernatural and other similar TV and animated series. Sam and Dean are not perfect and flawless superheroes, they are normal persons. They can make mistakes, they can be paranoid, confused or angry and they always argue with each other. Because of that, they are a more realistic type of protagonists and the viewers can relate more to their problems and challenges.
Sound and animation: 8/10
The animation is decent, but not perfect. Some things bothered me, like the awkward looking faces… but if you deal with demons, your face will probably look strange too. There are also some good ideas, like the excessive use of black ink in every frame, which creates nice contrast and the camera movement during some of the action or driving sequences, which almost makes you feel the action. The music includes some nice and slow themes, which create mysterious atmosphere. The good sound effects (like the Impala’s engine) and the occasional scary noises enhance the whole experience and should also be praised.
Enjoyment: 8/10
Supernatural has interesting heroes (and villains) and manages to create a scary and tense atmosphere, through good use of sound effects and music. There are a few great and very dramatic episodes with some interesting plot twists. The ending is above the average. It leaves the story open, but also gives some sense of completion. What hurt the whole experience is the lower quality of the stories created especially for the anime. I also expected Supernatural to be a bit darker and scarier.
Overall: 8/10
If you are not familiar with the series, the flashbacks in the beginning would be confusing and meaningless. However, little by little the background story and the important details are explained. Basically, Supernatural feels more or less complete on its own and watching the TV series before starting the anime is not an absolute must. It may even have a negative effect, as some fans of the TV series hate the anime, because “it is not exactly the same”. You can be smart and ignore them. Supernatural is a great series for any fan of the mystery/horror anime.