I'm one of those that saw the movie first and then read the book much later. When I do that, usually the movie gets in my head as "canonical", and it's up to the original book's quality whether I prefer it or not. If the reverse happens, nine times out of ten I'll prefer the book because there's just no way to express everything that's in a book in a movie format. But likewise there are things in movies that can't really be expressed in text. This movie is one of those. 
 To be quite honest, the framing device of the author's "abridgment" of a Florinese author's original mammoth tome based on his father's reading the "good parts" in childhood got old pretty fast. Well, if that were all it was, it'd be a pretty good framing device. But he took it way too far, going on and on like there were centuries of history for this fictional country and it was all based originally on something that actually happened and other real people like Stephen King were involved in this big framing story with not being allowed to abridge the second book, etc etc. 
 It's completely irrelevant to the actual story, which is why I have to say the movie is better. The framing was there, but only in the form of the father reading the story to the sick child. Great framing that doesn't get in the way of the story. 
 Plus the movie gave so much life to the characters on the page, from Fezzik's lovable stupidity and strength to Westley's chivalrous obedience early on and dashing competence later. A textual description could never capture Westley's expressions as he struggles with his near-paralysis before the castle break-in, or Inigo's intensity while facing the Count, etc. 
 On the plus side, the characters' backstory was very interesting to read in the book, something that just wouldn't fit into the movie except as a quick summary to understand their motivations. And of course, the story itself is just as full of adventure and true love as it always was, so I'm only deducting one star for the authorial intrusions. 
 And then there's the ending. Some argue that the ending was watered down in the movie, that it's a "Disney" ending because everything works out positively in the end for the heroes. But the book doesn't have an ending at all, really. Or rather, it has a fake ending and then the supposedly real ending, which turns into a fiasco of a cliffhanger, which then gets resolved through another set of implausibly daring and amazing feats of bravery and strength in the first chapter of the supposed sequel, which then ends on another fiasco of a cliffhanger. Given the framing of the story as the good parts of a book being read to a sick kid, whose later authorial voice keeps interjecting that of course it all comes out right in the end, this is completely contradictory and unsatisfying. I can't imagine a ten-year old being satisfied with this ending, in other words, and neither would I. But because the movie was plenty satisfying, I won't deduct any stars for the ending here.