"Harry Potter and the Deathly Hallows" was a fine book, and I think it works better as an ending to the series, rather than as a standalone book. I remember being happy to read it, and liking the familiar writing style and characters, while still being sad because this was the end of Harry Potter forever.
(Boy, was I wrong about that last one.)
The common complaint about this book is that nothing happens; it's nine months of Harry camping in the woods. And yeah, that's true, but there was character drama during those nine months, and I love that kind of drama. I also really liked the chapter where Harry spies on Dean and Tonks' father and a goblin. That was a nice chapter, even if it was mostly "people are giving important exposition right next to Harry without knowing it".
The premise of the book is that Harry has to destroy all of Voldemort's Horcruxes, only Harry has no idea what the Horcruxes are, or where they are. He just sort of aimlessly guesses things like, "Uh...maybe there's something at my parents' old house that I've never been to."
I was NOT a fan of the Bathila Bagshot sequence. Harry falls into a super obvious trap, and he wastes the perfect opportunity to kill Nagini the Horcrux, without Voldemort learning he is Horcrux hunting. Ugh, Harry, not good. I did like the sequence inside the graveyard, and I liked the sequence at the Malfoy's, even if that was just setting up for a very convoluted "Harry now has power over Voldemort's wand" twist at the end. Seriously. Harry has power over Voldemort's wand, because Voldemort killed Snape, who killed Dumbledore, who was disarmed by Malfoy, who was punched by Harry. That was pushing it.
I liked the Deathly Hallows plotline, although I don't think it really fit with the book. It was almost like they dumped a huge fetch quest on Harry at the very end of the series, right when he was already busy with a different fetch quest. At best, it's only ancillary to the book as a whole; "Harry Potter and the Horcrux Hunt" would have been more accurate. But hey, at least the Hallows were more relevant to the book than the Goblet of Fire was to its book.
The big ending to the book was great, and it was also great how they had an entire movie of basically nothing but the huge battle, starring all the characters ever. Great job! I was kind of hoping the movie would show a scene with Neville and Ginny's secret Hogwarts resistance group, which we heard about in the book, but alas, it did not. That would have been a good thing to include, to break up the long camping sequence.
Ever since book 4, I had decided that eye color had SOMETHING to do with the finale, because people won't shut up about Harry's green eyes. The killing curse is also green. Meanwhile, Voldemort has red eyes, and the disarming spell is also red. In the final battle, they use those spells; each person is casting a spell that's the same color as his opponent's eyes. And...it's just a coincidence. I read too much into things. Oops!
Oh, yeah. The only thing I didn't like about the big ending was the Snape redemption scene, which everyone already guessed at. I don't buy it. Snape was the main antagonist for the first five books in the series, and he was constantly awful to Harry. Just because he had a schoolboy crush on Harry's mother, that doesn't redeem him and make him a hero. If anything, that makes him obsessed with the past.
Complaints aside, it's a good book and a great way to end the series. I liked it!