Just watched Supernatural: The animation.
As a fan of the original live action show (up until season 10 or so anyways), it was an interesting experience, I liked it a lot.
Its got good Madhouse quality, you can tell they had some inspiration for Hellsing for some of the scenes.
The sub is largely better than the dub, you can tell they didn't get much of a cast together for the dub besides Jared Padalecki (and eventually Jensen Ackles for the last 2 episodes) who were the protagonists in the live action. The sub, meanwhile, has a pretty great seiyuu cast all around who gave strong performances, it seems the 2 seiyuus who dubbed the protagonists reprise their roles here.
Its interesting to see what aspects were changed for the animation and how the characters were changed. Dean was made to be a bit sillier/playful and more emotional, and Sam was made to feel more like a little brother than the original series had them as. Was a good change since it fits for animation, hard to have the same feelings like that when you see two adults in live action but in anime it just feels right for some reason.
They handle some story aspects better than the actual show did, and I liked the way they handled certain scenes. Music was good too. Animation was generally solid, there were plenty of cool moments and enough badassery to satisfy. The monster of the week episodes could often have a surprising amount of emotional punch to them.
Unfortunately the main story did feel a bit rushed towards the end, but it was pretty still well-handled all things considered. There was a chunk of stuff missing, but I liked it. I don't know if I would recommend it to someone who doesn't know what the show is, its hard for me to say if they explain the show well enough in context because it is a bit disjointed, but its not too hard a concept to understand imo, at least the first 5 seasons of the actual show anyways. That might be me talking in hindsight having watched a lot of it.
It was also a bit enlightening to help me realize how edgy live-action often is compared to anime. In anime they usually don't explicitly show shit like beheadings and bloody throat-slittings, often cutting the "camera" away to the head rolling off or something, but in this one they directly try to show the bloody/gory stuff more than most anime, and the live-action show might be more violent than this.
I'd recommend it, and would also recommend the live action of course if one happens to like the anime.