I'm sure only about 2% of everyone who went to see the movie actually read the book, so I'm not going to go into too many comparisons. Basically, the movie was a sugar coated gum drop version of the book and not at all what was expected. The book tone was dark and foreboding, but the movie was a make-you-feel-bad-be-scared-if-you're-parents yawn fest. But, once again I'm assuming next to no one read the book, because if you had, you would know that the book was very adult and would not be rated under R if made exactly the way it should have been.
If you were to go out and see the movie with no prior knowledge of it, it would be just mediocre to you as well. I wouldn't suggest seeing it in theaters. Trust me on this one.