Because I watched the movie version of Horns so soon after finishing the book (like that same night), this review will include a lot of comparison between the two. I don't normally compare mediums, as I prefer to enjoy each work for what it is (a reading experience vs a viewing experience), but I think there are few important differences to point out between Horns by Joe Hill and Horns by Alexandre Aja/Kieth Bunin.
The plot follows the book almost exactly (see my book review here for some details, or check out the Wikipedia page), except it removes superfluous scenes and several characters that would have only slowed the whole thing down and/or made it more confusing. Taking out Lee's mother and his treatment of her in her final days lets us focus on Lee's actions in relation to Merrin and Ig, while not showing us as much of his childhood makes him more trustworthy early on (and all the more manipulative later).
I found Glenna a lot more likable in the movie, and really all the actors did their jobs in making me care about their stories. Daniel Radcliffe is a perfect Ig, Joe Anderson is completely believable as Terry, the druggie musician, and Juno Temple captures Merrin's tenderness and passion. The effects were just enough over the top to keep this out of a strict horror genre; I'd call it a dark comedy/fantasy thriller.
If you're looking for something a little off the wall and want to reflect on what it means to be good and/or evil, Horns is your ticket. (Disclaimer: once again, I don't recommend this to anyone with a fear of snakes!)