For once, the critical reception is correct - this is probably the best-written, and best overall, Harry Potter book so far.
For the first time, the series truly feels like an epic, one big story. Things suddenly become very clear that were just there unconsciously before. Were you wondering why on earth Riddle's diary was important in Chamber of Secrets? It's all here. Why Ginny Weasley kept one-upping Cho Chang in Order of the Phoenix, and why Harry got a lump in his throat when talking to Ginny in that book? That takes off like a rocket in this one. The series now feels like a saga, one story told over seven chapters, rather than a series of vaguely connected seperate novels.
The central theme of this book is choice - who do we choose, and why do we choose them? It's a theme which has been present throughout the series, but really comes to the forefront in this book, and permeates all of the main plots.
The backstory on Voldemort is fascinating, and comes at just the right point in the series. We finally learn exactly why Voldemort and Harry are such opposites, and the choices they - and people around them - made to get them to this point. Would Voldemort have become evil had his mother not capitulated, and had survived to raise him? Had he choosen to heed Dumbledore's advice at Hogwarts, let him take him under his wing? Voldemort always had choices, always had a chance to turn back ... but he chose to isolate himself, cut himself off from human warmth and compassion, and love only himself. It cannot be a coincidence that we discover this in the book in which Harry falls in love - the difference between the two is made very explicit. One can love, one cannot. Rowling's device for Voldemort's immortality is wonderful, and demonstrates the pure evil of what somebody would be prepared to do in order to gain immortality.
The Snape-Malfoy murder plot is particularly fascinating, since the issue of Snape's loyalty is still left a little ambiguous at the end of the novel. Whether he is working for Voldemort or not at this point, he remains a complex character with mysterious motives, and nobody can predict which way he will go in the next book. Malfoy also, for the first time, becomes a complex characters, with conflicting motives. He is given a choice at the end - Dumbledore or Voldemort? It will be fascinating to see where Rowling takes this in the next book - perhaps Draco isn't just a token bully after all! The death at the end of the book is the most tragic and emotional yet - I dare you not to have a tear in your eye during the final chapter.
The romance is an absolute delight - sweet, funny, but very, very realistic. For anyone paying any attention to the foreshadowing in Books 1, 2 and especially 5, Harry and Ginny's romance has been a long time coming, and it has been worth the wait. The way Rowling has structured this romance, where the two have to grow up a lot emotionally before they are READY to fall in love, has been done sensitively, intelligently and realistically. Rowling's device for Harry's feelings - the "monster in the chest" - certainly brings back memories of what it is like to feel that initial rush of falling in love! Her ability to accurately describe the feelings of teenage boys is amazing! Ginny has been a fiesty breath of fresh air in the last two books, and it's great to see her coming into her own here. The romance feels real, and right for teenagers, but it also feels deep - their final scene together in the book is the most romantic thing Rowling has written in her novels, and is truly heart-wrenching.
Then there is Ron and Hermione. Quite probably the most obviously hinted-at romance in the history of English literature, they are still crawling towards the inevitable conclusion. Ron has to realise, in this book, that a relationship based purely on lust is not particularly enjoyable. There has been criticism of Hermione's character in this book, but if you have actually paid attention to her character in Books 1-5, and not been influenced by fanfiction or the films and elevated her to Super Logic-Woman, she actually hasn't changed. Hermione is probably the most insecure, and often emotionally unbalanced, character in any of these books. She nearly has a nervous breakdown over her school-work in Book 3, so her reaction to being spurned by the boy she has been in love with for three years seems very real, and certainly in-character.
This is the first Harry Potter book to end on a cliffhanger, and what a cliffhanger it is. This is Part 1 of 2 - "to be continued". Will Harry kill Voldemort? Will he survive? Where do Snape's loyalties lie? Will Harry and Ginny be reunited, and Harry allowed the live the happy life he wants? I, for one, can't wait to find out. JK Rowling has been getting better and better each time; if she carries on like this, the completion of this tale might be something very special indeed.