The gore and violence is over the top, and the mature subject matter prevented me from sharing the book with my 13-year-old daughter. Violence and darkness aside, Joe Hill's story is tight and well told, skillfully weaving flashbacks and present-day scenes, and inserting elements of foreshadowing to add just the right amount of complexity to the plot. Hill is the author of the acclaimed horror novel, Heart-Shaped Box (and his dad is Stephen King, which I just found out about 15 seconds ago).
I'm not familiar with Gabriel Rodriguez, the Chilean artist who drew Locke & Key, but his work is terrific. I have seen too many comics lately where the characters physical features vary from panel to panel so much that they are unrecognizable. Rodriguez's characters are extremely well-designed and consistent throughout the book, making it easy to figure out who is who. That might not seem like a big deal, but to someone like me, who isn't that great at recognizing faces, it's a big help. His depictions of architecture is stunning.
Most supposedly-scary novels and movies don't affect me, even though I enjoy them. But Locke & Key was both enjoyable and spooky. I highly recommend it.