First off, let's dispense with any sort of delusions and be up-front about something: It's nigh-impossible to review this series without comparing it to the live-action source. And given how many people are going to be coming to the anime series from the live-action one as opposed to the other way around, there's little good reason not to tackle this from the perspective they're probably wanting.
First off, the series is pretty solid, for the most part. For the most part, it captures the feel of the live-action show pretty well. And given that the individual episodes are half the length of the live- action episodes, that's pretty impressive. About 2/3 of the episodes are remakes of live-action episodes with some changes and tweaks made to maintain some mystery and surprise, and the rest are listed as 'original' but a few are clearly based off of specific episodes (with about as many changes as the remakes). The overall running plot is taken from the first two seasons of the live-action series, particularly with regards to Sam and Dean's search for their father and later the hunt for the Yellow-Eyed Demon.
While I personally was neither expecting nor desiring an exact word-for- word translation of the live-action show into the animated medium, I was admittedly a little disappointed that the animated series runs off of its own continuity. Changes to events are one thing; but at least one recurring creature from the live-action series clearly works under different rules in the anime. I fully acknowledge that this entirely a personal reference, but I was hoping for something a little more supplemental to the live-action series (like how some shows have novels and comics that take place in the continuity without necessarily implying an effect on the original series). A couple of episodes do this sort of thing pretty well, fleshing out the backstories of a couple of the other 'special children' that become more prominent as time goes on, but for the most part it's clear this is a different continuity altogether outside of the big events (finding John, chasing Yellow- Eyes).
The biggest adjustment to make is in the character designs. Some effort was made to make Sam and Dean's animated counterparts resemble Jared Padalecki (who also provides the English dub for Sam) and Jensen Ackles (who dubs Dean for the final two episodes only), and for the most part they succeeded. But other recurring characters (John, Bobby, Yellow- Eyes, etc.) are completely redesigned when they appear at all. And while I can respect that they wanted to be able to do their own takes on some of these characters, I personally feel that the series suffers for it. The animated Bobby, in design and portrayal, lacks all of the warmth and depth of his live-action counterpart and he fulfills no role on the series beyond simply being a source of information or a deus ex machina rather than a substitute father figure and confidant. The Yellow-Eyed Demon also particularly suffers from the animated portrayal, partially due to the lack of an actor capable of giving him nuance; his character design is so painfully generic they could have replaced him with a talking wooden doll. All he contributes in pretty much every scene in which he appears is a booming voice that reminds everyone that all is going according to plan.
But then, most of the characters who aren't Sam and Dean suffer a bit from the show's design aesthetic. You've got the animated version of Jared and Jensen and then everyone else (even characters adapted from the live-action show) is clearly pulled out of the 'generic anime stock character' filing cabinet at the animation studio. None of them are visually interesting with the exception of Missouri, and she's just too 'busy' design-wise. And overall, the contrast between the Sam/Dean designs and everything else is extremely jarring.
Also, while "Carry On My Wayward Son" is a good song, it's actually really easy to get tired of hearing it every episode as the credits music (where it pretty much kills the impact of its use in the season finale summary montages). And the fact that storyline-important details are left for a short scene after the credits is just obnoxious. Post- credits scenes are done in American television to add an extra punchline of some sort or what have you. But people who habitually skip ahead to the next episodes when the credits start are going to be left out of major plot points, and I'm honestly not sure why it has to be this way.
In summary: The series does a decent enough job of taking stories from the show, reworking them to fit in this format, and keeping things relatively fresh and surprising for people who've watched the live- action show. However, it does a crummy job handling pretty much any character who isn't Sam or Dean. Also, the pacing of some of the overarching story stuff is occasionally jerky and uneven. Unless you're a rabid collector of all things Supernatural, this is a rental at best. The people who enjoy the novelty of seeing reinterpretations of the show will find something to get out of this but hardcore canon purists are best avoiding it at all costs.
