There is a phrase that exists to describe works to which we are unreasonably attracted to despite their utter awfulness: so bad it's good.
The writing is laughable. This guy is like Dan Brown... in third grade. Words like "vermilion" and "phosphorescent" abound.
The story itself is not that bad. It's very cliche, but that's part of the appeal. With its thoroughly developed world, frequent homages to earlier vampiric lore, and an unwaveringly stoic hero, it has a very cult feel despite the lack of quality. Basically, the foundation of the story is fantastic. What is subsequently done with that foundation is anything but.
Would I recommend reading
these books? Maybe. The language is excruciating. It's not the kind of bad that flows because of its simplicity; it's laden with details of the worst kind (vehicles, weapons, and generally all machinery are described with useless precision), with purple prose, tangents, and poor translation. For this alone, waiting for an adaptation might be a better idea (so far there are two movies and the manga is in progress). But if you are interested in the inner workings of D's world, it is very much essential.