As a fan of the graphic novels, I was pleased to read that director Scott Stewart flew S. Korean graphic novelist creator Hyung Min-woo to America for consultation. It's pretty clear from the trailer and synopsis of the film that the screenplay differs significantly from the source material, but that's not necessarily a bad thing. From a story point of view, the Priest comic needed simplifying and a lot less backstory to work cinematically. I think the decision to change the original "demons" into vampires was a smart one, both creatively and commercially. Vampires are obviously part of the current entertainment zeitgeist and for a reason- they strike a curious mix of fear and awe in our collective psyches.
It also appears that we won't be getting standard vampires from the film. As the director said to MTV News, "Our vampires are not human. They’re not people." And when a vampire bites a human, the humans don't become vampires, but rather vampire slaves or "familiars."
One of the benefits of a Hollywood film of a relatively obscure graphic novel is that brings a lot more attention to the original material and to it's creator. Not only did Hyung Min-woo approve of the film's storyline, it appears he may continue writing more Priest volumes that follow the film's plotline. And, in my opinion, that's a good thing for comic fans and film fans alike. What does everybody else think?