"Dylan Dog" is based on a graphic novel of the same name by Italian author Tiziano Sclavi who placed his titular (anti-)hero in London. The movie puts the action in New Orleans - that at least was a good choice. The idea is fairly good: Dylan Dog (couldn't they have picked a better name?) is human, but the supernatural creatures in the city had accepted him as an arbitrator and helper ... until the death of his wife a few years prior to the beginning of the movie. Since then, he's been working as cheating-spouse-chasing private eye among humans. But the supernatural world drags him back in.
The effects aren't bad, but what really kills the movie is bad writing and seriously poor choice in actors: Brandon Routh is handsome and extremely weak in the lead, Anita Briem is just bad, Sam Huntington is weak ... A measure of how bad the acting is can be seen by who doesn't look bad: Peter Stormare phones it in and still looks okay doing it, Kurt Angle (with his professional wrestling pedigree) looks almost like an actor, and Taye Diggs steals the show. That's just scary (and not in the way horror movies are intended to be).
The story opens with Dylan being called to the scene of a murder and asked by the daughter of the murdered man to investigate. When it becomes clear that the killer was supernatural, he declines. But when his assistant is murdered and it's clearly related to the other killing, Dylan throws himself into the work.
The movie also falls down because of poor character interactions: Dylan's voice-over make it clear that Gabriel (Stormare) was his best friend: while they could have used Dylan's long disappearance as a reason for the animosity between the two, instead they frame it as being related to "that thing he did" which remains unexplained for the surprise value later ... and which it turns out that if Gabriel was really his friend, they would have talked. This is only one of many failings in the writing.
It turns out that there was another movie loosely based on "Dylan Dog," 1994's "Cemetery Man" starring Rupert Everett (only appropriate given that the graphic novel artist based Dylan's appearance on that of Everett). Unfortunately, that one is 20 years old and it's Italian-French-German comedy-horror - difficult to find in North America. Sadly, my normal source - Toronto Public Library - doesn't carry it.