Is there anyone who has not read the book yet, who cares? I guess I'll do a cut anyway, for the sake of length.
I saw the movie twice, once on Friday night and once on Sunday night, with different people. I liked it both times, the second time maybe a bit more (the seats were more comfortable the second time, which I think made a difference). I didn't think there was a definite arc to the movie--I didn't get a rush of catharsis or energy at the end--but I did like it better than the book. I loved the visualization of the text.
Of the books, this was the first one I had trouble finishing. The italics and all-caps and ellipses of Angry!TeenAnguish!Harry started to grate on me pretty quickly, even though he had some good reasons to be pissed off. I also feel the book was bloated, and the energy of the main scenes dampened by too much padding. There are many plots shoved together, and sometimes they obscure one another. For example, as my friend L. pointed out, in the book the scenes between Harry and Sirius, setting up their relationship, are so spread out amid all those pages that they're practically lost. In the movie, we got all of the scenes in which Harry and Sirius are together, while many other things were cut, so that plotline shone brightly throughout.
Most of the Order's plot is sacrificed in the movie. We meet the Order at the beginning, then quickly move on to Harry's visit to the Department of Mysteries. The movie focuses on the kids, instead, particularly on the Umbridge/Dumbledore's Army sequence, then into the final set piece. I liked the graceful, almost martial-arts movements of the adult wizards and the sheer scale of the magic in the battle between Voldemort and Dumbledore; it was a sharp and I presume intentional contrast to the kids' practice sessions.
There are a number of visuals in the movie that contribute to the viewing experience for a reader of the book. For instance, Tonks. She has only a couple of lines, at most, but we see her morphing her face to entertain Ginny Weasley, and we see her trip over something, showing her clumsiness. Lupin has, I think, only one line, but we see him once standing behind Sirius, another time overlapping with him as they sit at a small table, and a third time holding onto Harry, both grieving, after Sirius falls through the Door, then solo after Harry wrenches free. We don't see the portrait of Sirius' mother, but we hear her voice while Kreacher adjusts the curtain that blocks it.
I can't remember if this is in the book, but while Sirius is fighting Lucius Malfoy, and Harry helps him, Sirius calls out, "Nice one, James!" Ouch.
One visual detail I really liked was Ginny wearing patched pants, presumably a hand-me-down from an older brother. Also, Ginny was shown demonstrating powerful magic at least twice--a hint of things to come?
One thing I really didn't like was the Dumbledore and Harry Pietà at the end. I mean, really.
Alan Rickman was again awesome as Snape, even when he was barely on screen. You hear a line from him, off-camera, and later he has a scene with Umbridge, which he completely steals with exactly two words. He looms wonderfully in the Occlumens lessons with Harry. "I will penetrate...your mind." [ahem] Indeed.
Imelda Staunton ruled as Umbridge, especially when she was being sadistic. Her pink costumes beamed luridly from the movie's palette of dull blues and grays, but I don't think she needed that advantage. Her acting was enough. And the china cat plates. Mew!
Voldemort still has no nose. It looks pretty stupid. I refuse to blame Ralph Fiennes.