I've just re-readed it before reading the last part, so at last I'm going to write something about this one.
I have to read it again to be sure about my opinion, however I found it a bit less interesting than previous three parts, and it seemed to me a bit shorter. There is plenty of new secrets Harry learns about, and the book obviously seems to be a preparation for the seventh part, there are too many plots waiting to be untangled and explained. Waiting for the next book will be even more difficult!
I've read this book twice. It's shorter indeed, but in my opinion the best from the series. All previous plots are getting deeper meaning, everything starts to get cleared. I'm already waiting for another part :)
I also read this book and find it as fine as other five. Only it annoys me there are many girlish plots like Ron going out with Lavender or other things like this.