To me, books 1-4 feel like one long prologue, and 5 is where some of the pieces of the puzzle start to fit together. 6, then, is more of the wind-up to 7, which I'm hoping is 1500 pages long, as I couldn't quite believe that everything we as readers need to know has already happened.
While I still like the books much better than the movies, I'm finding that the movies - specifically the casting - have informed the way I read the books. The professors have all been pretty much perfectly cast, especially Snape (and Hagrid, too), and as such it's them that I imagine when I'm reading. I was always bummed out that Rickman/Snape never really gets that much screen time in the films - but as events in 5 and 6 will show, it's pretty obvious that he'll finally get his time in the sun, which will be awesome to watch.
Anyway, back to the book. I bring up the films because the first chapter of the book felt less of a source of exposition and more of a dare to the producers of the films - it would not surprise me in the slightest if they actually got Tony Blair to play the PM. (Either that or maybe Hugh Grant, a la "Love Actually".)
And the second chapter felt awfully wierd. JKR pulls lots of red herrings in the books - I was convinced that the half-blood Prince was going to be Harry's mom - and so I was highly suspicious of Snape saying the things that he says and pledging the things that he pledges. But, then again, Harry's been suspicious of Snape since day 1, so I didn't quite know what to believe, especially as Dumbledore trusted him implicitly and obviously seemed to be on a different, Gandalf-esque sort of plane of wizardry.
Ah, Dumbledore. I missed him in 5, and so I was incredibly happy to be spending so much time with him in 6, even though from the moment he entered the book - hell, from the moment in the first chapter where he was described as getting older and less reliable - I had a feeling that he was going to die. Dumbledore's death is earth-shattering - obviously - and during the funeral scene I, as the rest of the school, had a tough time really accepting it. And unlike when I read 5, I was not aware that someone was going to die in 6 - the big thing before I read 5 was trying to figure out who it was going to be. To be honest, Sirius' death, while tragic, never really moved me. I mean, I felt bad for Harry, but Harry had also been somewhat annoying in 5 and his mourning felt a bit too much like self-pity. Sirius had been Harry's buddy - Dumbledore was a lot more of a father-ish presence, especially since Harry'd known him for so much longer.
Harry, however, is fucking awesome in 6 - in particular, his telling off of Scrimgouer (or whatever the Head of the Ministry of Magic's name is) - not once but twice - is brilliant. "I'm Dumbledore's man, through and through." (It got a little dusty during the moment when Harry tells Dumbledore about this, and Dumbledore looks about to cry.) Gone is the whiny, pouty bitchiness - he's still as recklessly impulsive as he's always been, but he also appears to be a bit more comfortable in his own skin. His reaction to the insane, bloody curse he puts on Malfoy is totally genuine. Speaking of which, Draco has become a bit less evil and a bit more of a stupid pansy, which I guess is fine since Harry's got much bigger fish to fry. Harry's kicked Draco's ass too many times for it to really mean anything anymore, and even when Draco broke Harry's nose on the train it wasn't done with his cronies around, so the humilation factor wasn't as high.
The Ron/Hermione situation is dealt with, finally, and a lot less insufferably than the Harry/Cho arc. I certainly didn't see the Harry/Ginny thing happening, and I thought JKR handled it beautifully - the whole "She's my best friend's sister" / "But I think I love her" thing was just plain adorable, and JKR's description of Ron seeing the two of them kiss for the first time and silently nodding "if you must" was pretty much perfect.
As for the whole story arc - as I said, I can't quite believe that we're already at the penultimate chapter, and I feel like 7 ought to be 1500 pages long. I'm certainly as intrigued as ever about it - will Hogwarts remain open? and if so, is Harry dropping out? Is RAB Sirius' brother? Is Snape, in fact, really as evil as he seems, or did Dumbledore still know something that the rest of us do not?
I'm finding it hard to sum up my overall impressions about 6 - it's a good read but not as much fun as 5 was, nor is it as complete as 5 was; it's certainly the darkest of the series, and the ending is incredibly open-ended and unresolved. I finished the book and felt like I'd been punched in the stomach. If anything, 6 has made the wait for 7 that much more unbearable, since I feel like there's a lot more information out there. I'm hoping that we'll revisit the house where Harry's parents were killed - in fact, I'd bet that that's where 7 ends...