This is at least my fifth reading of this, easily my favorite book of the HP series. I get more out of each book upon every re-read, and this one especially. One of the great joys of the series, and especially of HBP, is the humor JKR manages to thread throughout the story, even as things are sad, or scary, or serious. The tiny references to Harry's burgeoning affection for Ginny, the way Fred and George manage to slip in one-liners even as chaos is erupting around them, it always feels like a gift from the author. This book, maybe more than any of the others, also really takes the time for character development. This is the book where Hermione becomes one of the great female characters in fiction, and where Harry stops being the victim of constant happy accidents and starts making his own choices. Plus it's just a really great mystery story! And ties in so importantly (obviously) with the overarching mythology.
I will always remember receiving this book the day it was published (July 2007, right?), a day I had to go to a wedding and also shop for a big dinner party. I hid the book under my seat at the wedding and kept turning the pages behind my ankles, only looking up at the ceremony every page or so. Then I had the book balanced on my shopping cart, blindly tossing vegetables and cans in as fast as I could. When I got home, I finished the rest of the book by 2am, texting with my sister, saying, "I'm on page xxx." We finished within a few minutes of each other, exchanging comments on what sobbing messes we were. Later that week, I was walking to lunch and saw a girl sitting in the sun outside of the bank (where she presumably worked). She had the book in her lap and her head in her hands, obviously on one of the sad, stressful final chapters. I wanted to just go over and give her a hug.