I finished the book several days ago (turns out I was close to the end, not the middle; the second half of the book is a collection of other, shorter, and believe it or not STRANGER stories from the author).
I liked it a lot better than the movie. I'd give the movie a B- and I'd give the book an A-. I love that the author goes to great lengths to create a scientifically explainable vampire. The story also has a big twist I wasn't expecting (which the movie doesn't even touch, nor could it considering how it portrays the infected). I also like how the book's infected act compared to the movies.
The movie OK, but the book was quite good.