The first mistake those who have reviwed this one so far have made is that they are not realizing what this book is. Simply put this is not a standalone book the way the others in the series so far could be. The story here is overture. This is the opening act to the final and 7th book in the series, and only that. This is (as said above in the subject line of the review is Rowlings "Empire Strikes Back". This is the book where the evil side wins. Whereas in almost all the previous books the bad guys have either lost outright at the end and been vanquished, or they have made gains, but at great cost (ie. Book 4: Voldemort killed Cedric, but Harry got away and wasn't supposed to). This is the first book where evil out and out wins. Make what you will of the Draco/Snape/Dumbldore chapter and it's conversations that have been talked over and over by fans.
Make no mistake, the stage has been set, and the pieces are moving for the greatest battle in the world of Harry Potter is about to come to pass with the next book. I understood this after reading Half-Blood Prince, and whilst it leaves you sad, and a little disapointed that the good guys are beginning to suffer horrific losses already, there is a scene similar to the scene at the end of "Empire", where you see through the darkness a little glimmer of hope. It's so tiny, but it's there.
This book truly is my second favourite in the series (though it may well only take spot # 2, because of my unabashed love for Book 3), and it sets the stage gloriously for what I know is going to be an all out, world-crashing, finale.
I prepare for the 7th book with the awe of knowing I have already read it's opening....with this Book 6.
Read it, and enjoy it on, this level. You won't be sorry.