I honestly wasn't expecting much out of this. A Christmas Carol with zombies? Not exactly promising. And yet, oddly, it actually works fairly well. What keeps this together, I think, is that McCann largely uses Dickens' original words, and what original text he adds he's careful to keep in essentially the same style. Not the easiest thing in the world to do. And he also manages to make an original, zombie-filled storyline fit into the basic story arc that one would expect from A Christmas Carol.
But it's still a zombie-filled version of A Christmas Carol. It's unnecessary in the extreme, and it doesn't accomplish anything that the original doesn't do twice as well.