I enjoy Hill's fiction in general, and I think Horns in particular is outstandingly meaningful and original. It's a love story and a murder mystery and the story of what happens when a mostly ordinary person begins growing horns that give him the powers of the devil, including learning everyone's darkest desires and being asked for persission to act on them. It uses quite a bit of Christian/devil imagery without accepting the dichotomy and taking either side: the closest to an ethical position that the story offers is that "sometimes there is no right answer," and then you have to pick the sin you can stand "and live with it."
The movie is an excellent adaptation, with all that originality and the strengths of the original in terms of character and plot. A somewhat less linear structure, with more flashbacks, works even better in a movie; the movie doesn't include some of the symbols in the novel, but it chooses the basic ones and uses them well. It's a nice combination: the movie is very visual, and it keeps the key points, but it isn't too literal in presentation of scenes from the novel (except perhaps at the end). Most of all, it gets the tone right, with a skillful blend of humor on the one hand and serious angst on the part of characters and you-want-to-look-away-but-you-can't scenes that make the viewer queasy on the other.
Overall, as one would expect from a story about human passions and sins, the movie presents a very intense two-hour experience. It's somewhat bloody, especially towards the end, but I wouldn't call ti a horror movie. It does explore the secrets of the human heart, but it does so in too understanding and hence sympathetic a way to create pure fear or revulsion. As the protagonist says when giving two police officers permission for their hidden desires, "It's only human."
Mood: teaching akmost 40 full hrs/week, glad to have a day off