Today I finished the second Harry Potter book, and I can say that it outdid the movie by a factor of approximately five.
A lot of things that were glossed over in the movie were more thoroughly explained in the book. It was as if you could see in front of you the producers/screenplay-writers sitting and cutting away everything but the stuff that would "look good on screen". Stuff that in the book takes up one or two pages had in the film become several minutes, and on the opposite side, a lot of things that I considered would have helped in explaining some of the events were nowhere to be seen in the movie.
(Though neither film nor book really explained the sudden appearance of a sword in any way that I deemed satisfactory, I guess every event can't have a logical explanation to it in a story about a wizardry school.)
There were two things that I felt after the movie that reading the book really made me re-observe: 1) the movie rushed through everything, what really happened during an entire school-year felt like it happened in a week, and 2) the movie emitted a lot more of the "Harry Potter saves the world again"-feeling than the book.
In closing, I would also like to mention that the appointed "bad guys" Slytherin seem too one-dimensional. Certainly if they were all bad they would be banished or something? I mean, there has to be some sort of quality about them, some reason why they're there besides being the nemesis of "good guys" Gryffindor. I just can't accept that someone is completely bad; revenge, megalomania, 'misunderstood genius', insanity, just give me something that motivates their actions. Whenever a character in a movie or book is presented as 'evil for the sake of being evil' I get first suspicious that I'm not being told the whole story, and if I'm not presented with anything then my suspense of disbelief drops like a stone.
I prefer to have my bad guys served with a portion of character, that's all.