This post should be read with the assumption that one has read Harry Potter and the Order of the Phoenix!
It ROCKS! I really really loved it. I thought they did a fantastic job with the screenplay - really finding important plot points and making what is really a very internal story very visible for the audience and screen. I'm liking the more recent movies, because they have more of a "real" feel to them - creating the atmosphere that the envelope on the desk there really could float up and start talking, for instance - rather than the somewhat separate world that was created in the first two films. There's a lot of nice one-liners in there, too, which I always appreciate. The kids are far more interesting now that they're growing up a bit. Tonks and Bellatrix LeStrange are cool and I wish they'd both had more screen time. Luna is just fun. Battle scenes are amazing, although I have to admit that one part of Voldemort and Dumbledore's battle at the Ministry totally had the fight between Obi-Wan Kenobi and Darth Vader in the original Star Wars pop into my head. We had a great audience - there were lots of cheers, boos, and applause for appropriate moments. Umbridge is just so...evil...you want to jump up and throttle her. The Weasley twins' escape is very fun, and of course, the Order of the Phoenix' entrance into the Ministry is just plain cool. I could go on and on, and I really want to see it again!
In all honesty, as much as I was excited about it, OotP is tied with GoF for my second-to-least-favorite Harry Potter book (my absolute least favorite has always been CoS, which I just can't get over for some reason), so I was a bit nervous. (Of course, part of that is still remnants of shock, anger, sorrow, and surprise that she killed off Sirius that I still attach to that book.) The only complaint I heard from anyone, which is somewhat valid but made me think "What the hell?" was that you really need to know the series, if nothing else at least know the end of the GoF movie, to really get what's going on. They assume, and I think it's a valid assumption, that if you're coming to see the fifth movie in a series, you at least know the general plot by this point, and they don't explain a lot of backstory. But actually, I think I like the movie better than the book. Sort of. Now if you'll excuse me, I have to go reread the book again!