OotP is my favorite of the HP books, and one of the very few books I've ever re-read almost immediately after finishing it. Because of that, it's the one whose translation to film has made me the most nervous about them "getting it right." I'm glad to say that, for the most part, they have.
I'm not going to discuss too much about the plot here - if you've read the book, you know it, and if not, I don't want to spoil it for you too much (and please explain why you haven't, and would want to see the movie anyway). It's fifth year at Hogwarts, and Voldemort has returned, but the magical establishment wants to deny it. As part of their campaign to do so, the Ministry of Magic installs a new Defense Against the Dark Arts teacher at Hogwarts, Dolores Umbridge, who creates an atmosphere of harshness and paranoia within the school. Inspired by the efforts of the secret Order of the Phoenix to combat the uprising of Dark forces, and encouraged as ever by his best friends Ron and Hermione, Harry begins training a small group of other Hogwarts students in practical defensive magic. Meanwhile, Harry tries to fight against his psychic connection with the returned Voldemort, which leads him, his friends, and members of the Order to battle against the Death Eaters in the Ministry of Magic itself - a battle in which, once again, Harry loses a parental figure.
I know the book is 800+ pages, and there were things that had to go in order to keep the movie from being six hours long. For the most part, I don't really have issues with what was trimmed, and the movie is very well-paced with nary a slow patch. There's no Quidditch, so we don't get to see Ron's athletic career commence when Harry and the Weasley twins are banned from the team by Professor Umbridge ("Weasley is Our King!"). A bigger concern is that there's really very little in the movie about the titular Order of the Phoenix itself, and that's disappointing. Other than Umbridge and Dumbledore, there's not much time with the Hogwarts professors - Professor McGonagall particularly is a glorified cameo, and again, that's just disappointing. The critical story elements are there, though, and the climactic battle in particular is conveyed successfully - parts of it that were hard for me to visualize when reading the book were very well translated to film, I thought. And the funniest scene in the book is here too - go Fred and George!
The major reason this is my favorite book, though, is the character development and its emotional resonance - Harry Potter as the angry teenager felt totally right to me (I've lived with an angry 15-year-old boy), and that was actually the part of the book-into-movie translation that made me the most anxious. When OotP was published four years ago, the movies had only made it through Chamber of Secrets, and I really hoped that Daniel Radcliffe's acting chops would be up to the task when the time came. Fortunately, they seem to be...all the young actors have developed well (and the changes in directors have probably helped that along).
I'm satisfied with this conversion of the book to a movie, but now I want to find time to read the book again!