I thought it was wonderful--the best yet. I was amazed how true to the book it was, not just because it pulled a lot of dialogue and images precisely from the book (Umbridge's horrible kittens are pretty much EXACTLY how I've always pictured them in my head), but also because it really captured the heart and soul of the story. Also, Imelda Staunton is nothing short of amazing. She was creepy and funny all at the same time.
My only complaints are:
1) It felt a bit rushed. I think there were a couple of places they might have been able to slow down just a tad more. At just over two hours, it wouldn't have hurt the film to have had a few extra minutes. But really, I was extremely impressed with what clever ways they devised to condense the story--certain decisions about changing the order of events worked really well.
2) Not enough Ginny. But that was to be expected. And really, the film has very sparse appearances by many of the great supporting characters, so it's not like she's the only one who didn't get her due. I have full confidence, however, that the filmmakers do understand her importance, and that she will get her chance to really shine in HBP as she should. There were certain...hints.
For the old-school snarx crowd:
Pay close attention to the behavior of Bellatrix Lestrange right before she is released from Azkaban. It...might remind you of someone. ;)