Orson Scott Card gave this movie a good review, which I read before I saw the movie.
Here's the part of the review with which I agree the most: "Without anyone taking much notice of it, Goldenberg has actually discovered what all those miserable little screenwriting classes pretend to teach: He has discovered how to tell a real story on film."
Goldenberg is one of the guys who wrote the 2003 Peter Pan movie with Jason Isaacs. That was truly a stunning bit of storytelling, I thought. And he's brought that same talent to OotP. For the first time, I felt as if I were watching a movie instead of the staging of a book. The narrative moved smoothly instead of feeling blocky and awkward, as if they were concentrating more on recreating the cool scenes from the book instead of recreating the story that was told in the book.
In the OotP movie, they concentrated on Harry's psychological and emotional state. This of course means that many scenes and subplots have to fall by the wayside except where they reinforce the main thrust of the film. I actually liked watching Radcliffe in this one, and Ron, very happily, was not relegated to comic relief except for some genuine jokes in the book, such as having the emotional range of a teaspoon.
They also did a good job of showing Harry's relationship with Sirius and, as Card says, the other "communities" that needed to be built. Luna served a wonderful role as the only person who seemed to understand, empathize with, and accept Harry. Some of the narrative shortcuts they took made sense and weren't disruptive.
The only disappointment I have is that I would have liked to see the scene at the end where Harry trashes Dumbledore's office. I guess they didn't do it that way, though, because they put the emotional climax in the Ministry of Magic, when Voldemort possesses Harry.
Also missing, and a good thing, too, was the cleaning of Grimauld Place and all of the rooms in the MoM. And Quidditch. They left out Quidditch, but it wasn't needed this time around.
Oh, and Helena Bonham Carter as Bellatrix LeStrange was brilliant casting. She was truly deranged, dark, and almost sexy, if she weren't such a loony tune.
It drives me crazy when reviewers slam the film for not having "enough magic." Because by "magic" they mean awe and wonder and charm and sweetness and discovery and newness, which was fine for the first movie: it was the exposition movie/book. But that's really background noise compared to the larger themes in the series, on which JKR concentrates more heavily with each novel.
OotP wasn't "faithful" to the books in the sense that it sought to recreate them. But it was VERY faithful to the spirit and thematic material, which, IMO, is more important for the movie than the special effects (of which there were many, nonetheless). This movie wasn't made to please the kiddies, it was made to tell Harry's story.