I'm still processing my opinion of it. Warning: spoilers beyond.
I no more think that Sirius is dead than I think that it rains peanut M&Ms. No body, kids, and likely still alive when he fell through that archway.
I was a little surprised by how dark this one got so early. Harry's forced self-mutilation by Dolores Umbridge was, I think, something I wouldn't want a child under the age of 9 reading unless I knew them to be an advanced, mature reader. Speaking of Umbridge, I did not feel she was adequately punished for her crimes, and the ending of the book felt a bit weaker because of that.
I liked that Ron joined the Quidditch team, and Ginny, too, in Harry's absence. I also liked how they've enriched Neville's role.
Harry's starting to feel a bit Rand Al'Thor-esque to me in certain respects.
I liked that James Potter was shown to be flawed, and that Harry's overly-idealized view of his parents was brought to a more realistic perspective. However, I felt that the learning of this information would have had more depth and meaning had it been used to bridge the gap a bit between Snape and Harry. I mean, they might not necessarily have become bosom buddies, but Snape at least understanding that Harry could see his side of things would have been much more interesting and preferable, in my opinion. Well, better than them just reverting to the "I hate you/I hate you, too" bit that's getting a little tired. They both deserve a bit more dynamic treatment.
Some folks have expressed that they hate the writing of Harry in this book. I'm still undecided about that. I mean, I can surely understand why he was so angry in so much of it.
When are they going to explain why Dumbledore doesn't give Snape the Defense of the Dark Arts position? I mean, if Dumbledore trusts Severus that much, it isn't making full sense to me.
I want to see Minerva McGonagall given the chance to kick some butt.
I liked that Lucius got busted, but I don't want to see Harry underestimating Draco. I think that boy's gonna be much more trouble than he's been.
J.K., I'm really bored with the whole S.P.E.W. thing. Give Hermione something better to do. I don't mind her being kind to the house elves, but there were segments with her going on about them that really didn't add anything useful to the book, the character, or the plot. A few little sentences are enough to keep that thread going without spending so much time on it.
The whole "Dumbledore's Army" bit felt a little X-Men-y to me.
More thoughts to come, possibly, but right now I need some lunch.