Rating: 1/2 banana (out of one banana)
This review contains spoilers, but only if you've never read the book.
My least favorite of the Harry Potter books has now become my second-least favorite of the Harry Potter films (just edged out by Chamber of Secrets, which was so bad I have put it out of my mind almost entirely). Maybe this was inevitable, as Order of the Phoenix isn't a fun book.
I'm not a purist, mind you. I understand that a humongous book does not lend itself easily to a 2 1/2 hour film adaptation. But, really, Goblet of Fire was nearly as long and the movie was easily twice as good. The blame here at least partially belongs to Michael Goldenberg, whose previous screenwriting credits include the very uneven 2003 Peter Pan and the so-so adaptation of Contact to the big screen in 1997. Then again, it could be director David Yates, whose directing credits include ... well, pretty much nothing I've ever heard of.
Steve Kloves, who wrote all the other Potter screenplays (and is slated to return for the next film), at least had a sense of pace and knew how to earn the payoff for scenes by building the suspense first. In contrast, Goldenberg simply jumps from one "memorable" scene to the next, with barely a moment left for character.
Without any development, moments that shone in the book, such as the Weasley twins' infamous exodus from Hogwarts, fall flat. They are essentially demanding a payoff that the film hasn't earned. Other scenes, such as Dumbledore's "not-coming-quietly" bit and the firing of Professor Trelawney, are so truncated that we have no time to enjoy them.
Conversely, scenes which could easily have been cut with little harm to the plot, such as the bits with Hagrid's giant brother, are given far too much screen time. I know that there's little reason to even have Hagrid in the film if you cut Grawp, but, really, there's little reason to have Hagrid in the film anyway.
And, of course, there must be arbitrary changes made. I suppose someone, somewhere thinks it's less "cinematic" if Dumbledore is always one step ahead of Umbridge, as he is in the book, but the net effect of removing any reference to his outwitting her by appointing a new divination teacher before she can and so on is just a weakening of Dumbledore's character, to the point that the film actually has Dumbledore snap at his students because Umbridge has gotten to him. If you've read the books, you know how completely out of character that is.
Edit: She Who Must Not Be Named reminded me to mention just how bad Michael Gambon is in the role of Dumbledore. Gambon is a fine actor, but just plain miscast, and has been for three movies now. While the late, great Richard Harris was able to project the right combination of kindness and power, Gambon seems unable to strike a balance. Gambon's Dumbledore is probably a more realistic person, but this isn't a genre where realism sits comfortably.
Snape, though well-played by Alan Rickman (as usual), is again relegated to scant minutes of screen time. His most crucial scene, involving memories of the past, survives in an extremely abbreviated form. Yates might has well have cut that scene too, however, since there is no reaction from Harry at all to the revelation that his dad wasn't such a great guy. We see virtually nothing of Snape's interaction with the Order, either. Instead, we are left watching a stupid bit with a magic ear on a string.
Still, the movie isn't without its bright points. Daniel Radcliffe is decent as Harry. He's not exactly Brando, but he's getting better. I'd like to say the same for Emma Watson, but Hermione hasn't made nearly as much progress as Harry in acting class. Rupert Grint is largely sidelined, since the entire prefect subplot is absent. I'm pretty sure Neville had more lines than Ron this time around.
Evanna Lynch hits the cute/weird balance mark for Luna Lovegood pretty well. She was probably the best of the kids outside of Radcliffe. Jason Isaacs actually manages to dial it back a bit as Lucius Malfoy, giving by far his best performance in the series. Imelda Staunton does a good job with the vicious Dolores Umbridge, though sometime in the middle of the second act, the screenplay starts to let her down. Ralph Fiennes is quite good as Lord Voldemort, chewing scenery like a half-starved termite.
The list of prominent actors who have about five lines in this movie is astounding: David Thewlis (Lupin), Brendan Gleeson (Mad-Eye Moody), Emma Thompson (Trelawney), and Helena Bonham-Carter (Bellatrix Lestrange) are all on-screen for a minute or three at most.
The movie's best moments are probably those involving the training sessions of "Dumbledore's Army" in which Harry imparts his hard-earned knowledge of Defense Against the Dark Arts to his fellow students. A substantial plot alteration regarding their eventual betrayal seemed fairly arbitrary, but other than that, it was alright.
Overall, I was a bit bored, a bit irritated, and only occasionally entertained. Your mileage may vary.