I committed a cardinal reading sin: I saw the movie before I read this book. To be fair, I was five--so not a lot of four-hundred page books were getting read then anyway. Nevertheless, I went into this story with as fresh a perspective as I could manage. Here is my opinion: the book is better.
For all of the humor in the movie, this book is funnier. Goldman's abridgment--which he painstakingly reminds the reader of in frequent italics, lest he get full credit for this story--keeps in a lot of Morgenstern's satire (some of it, the parts about trees and royalty, were cut out to keep with the story). Inigo, Fezzik, Vizini, and the whole gang are even more loveable on the page than they were on film. (Fezzik is my favorite.)
I don't want to give away any spoilers to those who have seen the movie, but there is more background given on the characters in the book. You get to meet Buttercup's parents, and see Humperdink's lunacy full-tilt; additionally, Fezzik and Inigo's friendship goes into more depth.
I am deducting a half-star for some Buttercup-Westley issues. Are they precious? Yes. Do I want them to survive their plight? Of course. But the slap was excessive. (Those who have read know what I'm talking about.) I was also annoyed by Buttercup's stupidity. I mean, sure, she's helpless and this is a satire about high adventure, but does she have to follow everyone's orders? (Granted, I still love her, but come on.)