(Updated below.)
I finished book six a while ago; just haven't gotten around to posting until now. Good Lord it ends on a depressing note. We can only hope book seven will bring relief.
My favorite books are still one and four, I guess. I can't remember laughing since book four. Maybe there were some funny bits; they just didn't stick in my mind. Book four was a turning point; there hasn't been a happy ending since book three. The end of book five could have been happy--Harry had been proven to be telling the truth, Umbridge was ousted, Dumbledore was back in charge, and yet, because of the death of Sirius, there was no celebratory feel to it.
What a great idea Rowling came up with when she wrote the first book. A kid living in miserable circumstances is plucked out of his life and dropped into a world of magic. Any child could picture themselves at Hogwart's: it wasn't long, long ago or far, far away: it was here and now. The world of magic was so close you might pass an entrance to it every day and not know it. Why, you could get there from King's Cross Station...just there...there between platforms 9 and 10. Hogwart's was a safe place, a refuge, a sanctuary for every unhappy child.
Not anymore; not since book four. I confess I don't like what Rowling has done with the story. Nobody could read books five or six and not be depressed by the endings. But I'll withhold final judgment until I read book seven; maybe she'll make it fun to read again, in the end.
P.S. I meant to add that Snape's having murdered Dumbledore doesn't yet mean he's beyond redemption. It's possible that Snape realized Dumbledore was dying from whatever he drank in the cave, and that he, Snape, couldn't save him. Thus he may have hastened what would otherwise have been a slow and painful death, while fulfilling his Unbreakable Vow and getting Malfoy out of a jam at the same time. I still have hope for Snape, I can't help it.