Horns is Joe Hill's sophomore effort, after Heart Shaped Box, which I enjoyed very much. Unfortunately, I didn't enjoy Horns nearly as much. Although individual parts are well-written, the whole didn't hang together for me.
One of Hill's strengths is his characters. The major characters feel alive, especially /Merrin, the love interest of the piece. Through the course of the novel, she seems to grow from an idealized image to a fully fleshed-out person. On the other hand, he doesn't give nearly as much attention to his minor characters. They're there to give protagonist Ig something to react against, and aren't much more than props. (This includes Ig's father and mother, who should by rights have been ore important characters).
My bigger problem with the novel, though, is structural. Ig wakes up one morning with horns growing out of his head, and finds himself gradually adopting the powers of the devil. But it's pretty random -- why does it happen? Why does Merrin's crucifix block his power, but others don't? Why do people start telling him what they think of him -- that's not really a power associated with the devil? I get that fantasy novels don't always have a good rationale behind them, and horror novels even less so, but even Ig wonders about some of these questions, foregrounding the fact that they make no sense. Yet this also isn't presented as some kind of Lovecraftian cosmic horror, where you'll never understand what's going on because your brain is just too limited.
Individual sections work pretty well in isolation. Hill knows how to write a compelling scene. But, unlike Heart Shaped Box, which felt satisfying on every level, Horns didn't really deliver for me.