Okay, let's start out with the most surprising detail before we get to the main text of the review: Voldemort looks quite cutting in the black suit, shirt, tie combination. I'm dead serious. He has a damn good tailor, whoever he is.
The movie itself was satisfactory, if not outstanding. The Order of the Phoenix was one of the two weakest books (along with Chamber of Secrets), and if anything it seemed like an even worse candidate for film adaptation. But, perhaps recognizing their constraints, it actually came to life about as well as could be expected. As usual with the HP flicks, people without a background in the series (i.e., at least having watched the other movies) will be completely lost, but these movies were always made under the assumption that its viewers were fans already. Within that framework, the movie rolls along, doesn't go for too much, and most importantly, doesn't focus on the parts of the book that would spell disaster for its cast (in other words, it didn't make Daniel Radcliffe act).
One of the movie's better lines is when Hermione Granger tells Ron that he has "the emotional range of a teaspoon," but honestly, that line would have better been spoken to Mr. Potter. Daniel Radcliffe has always been limited by the fact that he can't express more than one emotion, and this script must have taken that into account. In the early movies, the magic emotion was "surprise." In this movie, surprise is out, and glowering is in. Boy, can Radcliffe glower. And to be fair, glaring and steaming and smoldering is how Harry spends most of Book Five, which is one of the reasons it aggravated me so much. Mercifully, the movie's producers decided not to focus on that theme of the book, and so for most of the movie Harry is rather expressionless. Which is where he is at his best. Aside from that, little of the acting was worth note. The other main characters manage to hold themselves reasonably well. Dolores Umbridge was, in my view, overplayed, but my brother said she hit the target dead on. Luna Lovegood flirted with being really well done, but was a bit too affected even for such an outlandish character.
Shorter than its predecessors, the movie still has pacing problems, tending to drag when the producers get CGI-happy. The reverse problem, of course, is that the movie feels perpetually rushed--the product of cramming a book the size of Order into a watchable movie. Major plot events occur in a single scene, without expounding, lending the whole show a "blink and it's gone" feeling. Prioritization would have helped here--there were a lot of areas that got short-changed, and a few that could have donated some precious minutes. The time spent showing Harry training his DAers, especially, could have been shaved and redirected to prevent the movie from being spread so thin. The final battle in the Ministry of Magic got the right amount of time and rang reasonably true (Bellatrix Lestrange was played beautifully, by the way), so kudos there. However, as it went on, it began to over-reach itself. Too many flashing lights, too much wanton destruction, even the summoning of a monster. It felt like it was cribbed from a Dragon Ball Z cartoon, or a Final Fantasy "limit break" animation. If, after defeating Voldemort, the soundtrack had broken out into FF's "victory" theme (da-da-da-daaa-da-daa-da-daa-daaa!), I wouldn't have been the least surprised. This shouldn't happen.
All in all, it was a fine movie, worth seeing, but nothing ground-breaking or earth-shattering. Given how weak the movie series started, I'd be quite pleased if it managed to find its niche as solid but unremarkable, as it seems significantly more possible for the movies to get worse, then to get better.