I warn you right up front here--this book will break your heart.
When the fans were waiting for this book to be published, JK Rowling told us to expect a death in this story. The fans went wild trying to imagine who it would be. And I will tell you right now, that the death still has ripples in fandom, even five years later.
And even beyond that? This story is not so much dark as horrifically painful. If I had not been waiting so long for this book, I cannot help but wonder if I'd have made it through the story at all. Between the wizarding government and the Death Eaters, it's a wonder Harry manages to stay sane in this one. Especially after the Death.
There are good moments to this story, and some fun additions to the cast of characters--Luna, Tonks and Kingsley in particular. But if you go into this book expecting something like Prisoner of Azkaban or even Sorcerer's Stone, you will be sorely disappointed.