Definitely more graphic and unhinged than the movie version, but the movie did it justice. In the book, you don't "see" quite so much of the Exorcism, but you do get the effects of it on the people around them, and that's pretty rough to see. Especially Regan's interactions with others when she is in a scene. 

But I really could've done without so much of the detective. He's obnoxious and he rambles so much that I don't even remember why he's there half the time.