Let me first say that 90% of the people who will come to this page should ignore most of the bad reviews for this film. After looking through them, they seem to all have one thing in common - anger that this doesn't seem like a faithful adaptation.
I'm willing to bet that most of the US audience for this film has never read the Dylan Dog comics, and, like me, had never heard of Dylan Dog before hearing the title of this movie.
Now, putting aside the negativity that comes from fans of the source material, I think most fans of the genre will like this movie. It is very in the vein of things like Joss Whedons Buffy & Angel as well as sharing some similarities to things like Constantine and the Dresden Files book series. Let's be honest, this is urban fantasy. Most people know if they enjoy the genre or not.
The film does a decent job of keeping a film noir tone, and using noir tropes to it's advantage. Routh is good as the title character, doing a nice job of trying to be the tired detective without being a weary stereotype. Sam Huntington is great as Marcus, who I think ends up stealing the movie because he really has the lions share of memorable moments.
Yes, chances are you'll figure out the who done it before the movie reveals it, and you'll figure out impending double crosses from the moment characters are introduced. That's the danger of being a genre film, but it doesn't detract from the fact that the movie is really fun, and very funny in quite a few places.
If you like things like Buffy, Constantine or Harry Dresden, give this movie a chance cause I think you will probably end up liking it.