I thought it was the strongest book of the series. It took balls to kill Dumbledor in the last book, and I think she spent a lot of her credit. She owed her readers a lot after that and I think she delivered. I did find myself getting irritated with the long-winded inner struggle Harry was having about Dumbledor's flirtation with the dark arts based on something Skeeter wrote. I also (and this applies to earlier books as well) tired of the good guys using nothing but stunning spells over and over and over and over and over. How many people does Bellatrix have to murder before you think to yourself, "Ya know, maybe we need to put this lady out of commission permanently if we don't want to continue losing loved ones to her hands."
My wife and I read the book together. I do all the reading for the most part because my voice never gets tired, but she can't go more than half an hour or so. It was a fun read. We had guessed that Harry was a horcrux, and I suggested that was the real reason Voldemort couldn't kill him -- because only Basalisk venom and super-fire or whatever it was called, can destroy horcruxes . . . but then we counted them up and they were all accounted for so we dismissed that suggestion. I also guessed that Snape was the actual master of the elder wand before Voldemort had even taken it out of Dumbledor's hands, but the Malfoy business never even began to occur to me (though, I question why the elder wand would give its allegiance away simply because another wand was won from its master. But . . . it's her world, and it made for a great twist. It made for a believable and satisfying final showdown too -- I always wondered how the hell she was going to pull off having Harry beat Voldemort when, up to the end of book 6, his magical skills were absolutely pathetic. I still wonder where all the adults learn all their fantastic magic in the books. It ain't at Hogwarts. By the time kids graduate from Hogwarts they can't do much besides making light shine out the tip of their wands and fetching things from across the room.
Anyway, really good book. I'm excited to see what's next out of her. BTW, if you want something kind of along the same lines you should check out the Artemis Fowl series. Really good stuff.