To my extreme joy, I found a story that I can recommend! Yay!!
The story is "The Walking Supernatural" by daksgirl.
Basically, it's an alternate S6 of SPN and an alternate (though close to) very early S2 of WD. It involves the dead coming back to life, not through a virus, but through a physics hiccup caused by the collider in IL. They'll be very glad to know that they beat the Swiss in finding the 'god particle', right up until they get eaten and then taken over by an anti-life, dark beast.
With the world on the ropes... again... it falls to the Winchesters, Daryl, Dale and Andrea to fight their way through the horde in order to reverse what was done with help from a reduced-power Castiel and Crowley. Yes, Crowley plays the hero! Well, y'know, he does have that whole motivation of Hell being endangered, but still... hero!
Friendships are forged, Cas and Dean slash it up, and Crowley shamelessly flirts with Bobby to his (mostly patient) consternation.
The Good: The character moments are wonderful with strongly written personalities for Dean, Sam, Cas, Crowley and *gulp* GOD. There is also some really nice work on capturing Rick's and Carol's voices and I loved the hunter-partnership built between Sam and Daryl.
I really enjoyed how Sam and Daryl form a friendship, that turns into a new Hunter-partnership when Dean pairs off with Cas.
Strongly written fight scenes are important for this sort of fiction, and our writer does a great job of describing the oozing, black-blooded, rotted zombies going hand to hand with our heroes.
The Bad: I don't feel the need to put anything here. Yay!
Other Thoughts: Well, I do think that the WD characters are mostly given short shrift (excepting Daryl), but it isn't enough to place it in 'the bad', it's just noticeable. I believe the author was simply more comfortable with SPN, than The Walking Dead.
I also think that the WD characters accepted the demons/angels thing a little too easily, even with a zombie apocalypse surrounding them. It's not in 'the bad', because I'm sure that the story would've suffered if we had to get bogged down in getting everyone's trying to deal with the fact that Cas was an angel and everything that would go with that but I have to admit that I'm disappointed that we didn't get any scenes of Castiel and Herschel having a sit-down chat. That just struck me as a lost opportunity for a powerful scene.
The Score: 4.0 outta 5 stars ... a great story!