I went to see the movie yesterday. I should preface this by saying that I have not read the book, and one of the reasons why I wanted to see this was to see how Seth Grahame-Smith wrote a screenplay to source material I wasn't previously invested in and possibly see why he was selected to pen (or at least tweak) the Dark Shadows screenplay.
While I felt that the concept had a few clever concepts to provide "context" to things like the Lincoln-Douglas debates and his son Willie, I pretty much felt like this movie was a hot mess. It kind of felt like it couldn't decide if it wanted to be an action movie or a thoughtful reimagining of historical events, and therefore fell short of doing either. They never really explained how their vampires were free to roam the earth in daylight, how some vampires seemed to be able to largely control their urges or any restrictions on killing them. There was something fairly provocative about the naming of the primary antagonist (I don't want to say anything more for fear of spoiling anything), but it was never delivered on. Finally, there were a couple of ostensibly surprising plot twists that I found fairly transparent.
The long and the short of it is, I don't understand what it was about this that would have prompted Tim Burton to invite Grahame-Smith to tweak the Dark Shadows script.