An impossible-to-film book, like Fitzgerald's Gatsby, but Wyler and Goldwyn did a pretty good job of it. They turned it more or less into a love story, and a kind of a rags-to-riches, ugly duckling, class warfare thing. Which is fine, it's all there in the original book, but none of it is what the book is about. Not exactly. It's always struck me as funny though to see some elements that could not be quite cut out of the adaptation, but which really only make sense in the context of the book - the occasional mentions of Heathcliff's devilish nature (in the movie just meant to refer to his being a jerk and vindictive, I guess - in the book it's arguably literal), Cathy's spiel about "I am Heathcliff" (in the movie all that talk is meant to play as love talk; in the book it's a lot deeper and more complicated).
I hate to make this a book vs movie thing, though. The movie could never have won such a contest. I liked Olivier's performance for the most part, mostly due to his physical presence and his look. He looked the part, and standing there, had the posture for the part. His acting choices were often less in line with what the character ought to have been. Heathcliff was a character who shouted his passion. He was simmering. Even his revenge on the Earnshaw son, retained in the movie, was a passionate sort of thing. I felt Olivier chose too often to be restrained and understated and quiet. This character was not that.
I didn't like any of the other main casting choices besides perhaps Niven, who though he isn't my idea of Edgar Linton, played the part admirably enough. This Cathy was not enough at all. And her wide-eyed finale was fortunate to be caught up in the movie's momentum and in Olivier's performance, else it would have been laughable.
All in all it's a good film on its own, which is the best way to judge it. It's just difficult to separate it in mind from one of the truly great books ever written. I don't think it was an earnest attempt to really film the book, nor do I think such an attempt has yet been made by anyone, nor do I think such an attempt could possibly succeed. But I give them credit for at least getting a good movie out of it, which so many have failed to do with Gatsby.