The book Harry Potter and the Order of the Phoenix is my favorite in the series so far (I have not yet read Harry Potter and the Deathly Hallows). Let me also say that the story becoming darker is, in my opinion, a good thing.
The Harry Potter movies have become progressively better with each installment—until this movie, that is. Harry Potter and the Order of the Phoenix is the least good of the movies in the series thus far. Believe me when I say that I wanted the movie to be great—more than great, in fact. I am aware that in the process of transforming a book to a movie things are added, changed and/or removed, but I expected the movie to be better than this, to be at least on a par with the fourth movie, Harry Potter and the Goblet of Fire.
The cast generally performs very well, just as expected. Imelda Staunton is excellent as Dolores Umbridge, but the character did not evoke the same strong emotions in me that I had when I read the book. I am definitely not a Harry Potter expert, but in my eyes Evanna Lynch does not just play Luna Lovegood, she is Luna Lovegood; whenever I saw her and heard her voice it was my mental image of the character that I was seeing and hearing. Of the regular cast, Alan Rickman deserves special mention, as always, for his shining performance as Snape.
My biggest problem with the movie is the pacing: there is a lack of flow. That may not be an issue for people who have the book in fresh memory (I read it a long time ago) and can easily fill in what is missing from the movie, but it is nevertheless a weakness as the movie should work coherently on its own. I agree with the following from Mick LaSalle’s review of the movie:
Taken as a motion picture, the new "Harry" comes up short. But taken as a visual aid to the experience of reading a book, the new "Harry" does its job.
As it stands, Harry Potter and the Order of the Phoenix feels incomplete: a highlights version of the movie it should have been.