I saw this one last night. I had read the novel, and enjoyed it greatly, so I was a bit predisposed to like the movie.
And I did. Like the novel, it could have been a one-joke campfest, but they cared enough to strike the proper balance between the silliness of the concept and the seriousness of the setting and themes. And it worked overall because the idea of vampires as dark forces in Civil War-era America actually works surprisingly well with existing history.
It wasn't perfect, as some of the action scenes dragged a bit (I'm among those who wasn't a fan of the horse stampede fight) but I found it a solid B+.
It's a shame that two of my favorite movies this summer - ALVH and John Carter - have both flopped.