Having given some time for those likely to care most about such things to finish reading the book for themselves, I'm going to avoid any pretense and simply write an in-depth review with spoilers. Feel free to include spoilers in any comments.
If you haven't read it and don't want to see spoilers, consider yourself warned.
I can't by any stretch claim to have come upon the Harry Potter books "before they were fashionable", but given that I came upon the series back in 1999, I suppose I do at least have the dubious distinction of having been reading them since back when most writers in Catholic circles who had noticed them at all were still muttering darkly about temptations towards the occult and expressing shock (shock!) that a book about a bunch of kids at a boarding school often involved main characters breaking rules and such. These days things have moved around a bit, and while there are certainly still some dark mutterers out there, there are others who have gone to the opposite extreme and consecrated J. K. Rowling's series as a deeply Christian narrative second only to Lewis' Narnia books. Meanwhile, out in the real world there are roughly three groups of people: a large group who have found the series quite enjoyable, a somewhat smaller group which finds it boring or annoying, and the largest group at all which simply doesn't care, for one reason or another.
Now that the last book of the series is finally out, it's possible to take a look at the series as a whole and attempt to come to some conclusions. From that vantage point, it seems to me that Rowling has, in the end, written something quite good. And yet, at the same time, even in this last of the books (which I think was in many ways the strongest of the seven) there are some noteable plot/writing problems. Which leads me to take a step back and look for a moment at what exactly one may mean by "good" in relation to a novel.
Good can mean:
1) Fun: Escapist, inventive, scary, a who-done-it or what have you, these are the basic "good reads". Opinion can vary a great deal on these, since their enjoyment is sometimes very much a matter of what you're in the mood for. I tend to be neither a horror nor a romance reader, and so the mere existence of horror or romance does not make a book "fun" for me, but for some it does. Back in the day I read a lot of fantasy and science fiction, and with many of those "fun" books the attraction was essentially that they provided a different and evocative world to live in for a few hours. Some might say that to list this under "fun" is to downplay it too much, but I think I would also put in this category works that are "good" because they provide images that are lastingly evocative, like the lady with the birds in Bleak House. Such images may turn around and be examples of great prose, thus meeting 2) below, or reflecting life in some very clear way, thus meeting 3). But other times, the image, however interesting, goes no farther than being interesting in its own right. (Examples of this from the Harry Potter series might be the Dark Arts teacher in Philosopher's Stone who has Voldemort on the back of his head, or the pen in Order of the Phoenix that writes in one's own blood.)
2) Well written: Some books are just a pleasure to read no matter what is going on. Pick up Man Who Was Thursday, Emma, Barchester Towers, Three Men in a Boat or Brideshead Revisited and you stand a very good chance of opening it to a page which is simply a pleasure to read aloud, though each in a very different way. Whether by memorable turn of phrase, brilliant fit of words to subject, or ability to evoke things that few others can, some authors write in a way which is worth reading no matter what they're talking about.
3) Saying something about life: At its best, a novel is like life distilled. Extraneous detail is passed over, and only those feelings and events which best convey the essence of what is being described are included. Which books we see as such depends on what we believe about the world. Somewhere out there, there must be someone who thinks that The Stranger tells you a lot about the human condition, but I don't. For my part, I see books such as Brothers Karamazov, The Lord of the Rings, The Great Divorce, The Secret History and Keep the Aspidistra Flying as each in their ways displaying important elements of the human condition. In a good book, this is done in such a clear way that one can say, "It's like when..." and reference the scene to a fellow reader, thus conveying successfully what you mean much more clearly and succinctly than you might be able to in quite a few paragraphs of non-fictional description.
Having laid these three types of "good" out, I would tend to argue that the Harry Potter books are, in the end, good in the first and third senses. However, they are far from so in the second.
Starting with the first form of "good", Rowling has brought a whole host of images and ideas to delight, horrify, fascinate, and suggest over the course of her seven books. She's provided us with one of only two examples I can thing of where an imagined sport seems so fascinating you wish you could see it done for real. (The other being the zero-g wargames in Enders Game.) She's given us a visual example of the saving power of a mother's love, a darkly evocative image of how life can be extended by feasting on the blood of innocents, and a train that leaves from an invisible platform located between two real ones. And that's only in the first book.
Throughout the series, Rowling's wildly inventive imagination provided readers with a wide variety of situations, things and places which catch the fancy. 'Wildly' is the appropriate adverb here; although Rowling excels at coming up with images that range from the amusing to the terrifying, consistency and thoroughness are not her strong points. There are fairly major things about her world that simply don't make sense (like how it's somehow necessary to keep the knowledge of magic from Muggles, and yet it's clearly an open secret for people like the Dursleys or Hermione's family) and some things on which major plot points ride that are not treated consistently (such as the question of whether dead souls can "come back" or communicate in any real sense with the living -- we are told not, and yet Dumbledore is apparently forming now plans and issuing instructions for nearly a year after his death via his portrait in the headmaster's office.)
This leads into the question of good writing. Let's face it, the craft of writing is not Rowling's strong point. Her prose has a fairly middle of the road, transparent quality. Her dialogue can be a bit on the basic side, but perhaps that's simply a result of writing about characters in their teens. (Which of us really said much of anything quotable at that age?) Her real problems are with structure, and since it's the final book that I'm reviewing, I'll stick to discussing the writing in that one.
Perhaps some of the things that bothered me simply resulted from on-the-fly plotting near the end of writing a long series. For instance, we get a huge amount of backstory and motivation for Dumbledore in this last book. Generally, I thought that the story we got was interesting, though the character of Dumbledore who we know by the end is a proud and controlling creature, much less likable than we imagined before. This perhaps makes some things (such as his planning of his own death) more explicable, and less of a moral issue, than if he were the unquestioned good-guy and omniscient narrative force that he was in the previous books. When Dumbledore tells Harry near the end that Harry is the better man, we can believe him.
However, I think it's honestly too much change in a major character to bring up suddenly in the last part of the last book. It seems too much like a last minute attempt to do "grown-up" plotting with conflicted characters and deeper motivations. (And, indeed, I'm pretty sure it was just this sort of last minute attempt -- unplanned before this book was written.) To make things worse, Dumbledore continues to behave as if he were some sort of omniscient narrative force, rather than a all too proud and fallible character. His plan, formed apparently before his death in Book 6, plays out in unbelievable complexity and yet without a hitch throughout the final book. You just don't get away with making plans that complex if you're something other than an artificial token acting to move the plot along. And yet he does.
Another problem is just plain narrative structure. It's been Rowling's policy, and generally a good one given the type of books these are, not to allow the reader to know anything major that Harry does not. However, this means that in the last hundred pages who have two entire chapters where the action screeches to a halt and we get lengthy passages of exposition, first from Snape, then from Dumbledore. It's all interesting stuff to know, but from a writing perspective, it's clumsy. Far too much time is spent (for any stage, much less so late in the arc of the plot) on getting these things out via flashback and long stretches of dialog. I was interested in what I was learning, but I was groaning internally at the same time.
All of which is starting to make this last novel sound like a pretty poor showing. (It doesn't help that most of the problems are near the end.) And yet it's not. And that, I think, is because on the third type of goodness, Rowling comes pretty close to hitting a home run.
Throughout the series we've seen a strong contrast between the Death Eaters and the Order of the Phoenix (as I think one may call the 'good guys' in general, since the Order goes back to the original war against Voldemort.) Voldemort and his death eaters not only see the lives of Muggles and non-pure blood wizards as worthless, they also have a destructive desire to cling to life and power in this life. Voldemort has gone so far as to split of fractions of his soul and embed them in various hidden objects so that he can't be fully ejected from the world when his body is killed.
The Order, on the other hand, exemplifies a rightly ordered love for life and for friends and family. Harry survived Voldemort's original attack (and Voldemort's body was destroyed) because his mother's sacrifice of self to save him provided him with a protection against Voldemort's powers. The contrast comes into full focus in this final book. One of the first major events is the wedding of Bill and Fleur; and Tonks and Lupin got married just before the books action commences. Harry, still only seventeen, yet no older than many another young man in the real world, sent off to face the likelihood of death, is clearly feeling a war-time urgency about his own relationship with Ginny.
The clouds are gathering quickly in the wizarding world, and as wedding and war plans are both made, the characters clearly understand there's a likelihood of people ending up like Harry's parent's, or like Neville's. Nor is this just some pre-war sex craze. (In a move by no means standard in books for teens, the Harry Potter books are completely free even of references to premarital sex.) It's specifically marriage and family that these young people who may not have a future are seeking.
This makes all the more powerful the moment, near the end of the book, when Harry realizes that he is going to have to offer himself up to die in order to save the rest of his friends. To his great credit, the knowledge that this is the only way to defeat Voldemort turns into the determination to do it almost without consideration. And as Harry secretly departs on what he believes to be his last, brief journey, his thoughts are on those who have died already and on the family that he and Ginny will never be able to have.
What follows as Harry offers himself to be killed, humiliated, and then have his friends slaughtered anyway -- and his return and defeat of Voldemort -- could only get more Christological in its symbolism if... Well, if it was The Lion, the Witch and the Wardrobe.
This theme of having and protecting family is underscored again in the final battle with Mrs. Weasley's avenging angel moment, and ties off nicely with the epilogue. There are those who've seen the epilogue as a pointlessly "cute" add on. These are probably the same people who can't imagine why the end of LotR "wastes" all sorts of time talking about the Shire and Sam's family.
It's surprisingly unusual to find a book that sees family, that drama which is most common of all, yet ever unique in its details, as the foundation of happiness and peace. Harry Potter does it well. And in the end, I think that's one of the reasons why I found the final book so satisfying.
Is it "a masterpiece"? Is it literature? No. It's not well written enough for that. But it is a solidly good set of books, with a trove of fascinating imagery and a foundational set of human values that rings true. I'd have no problem recommending the entire series to kids from twelve or so on up, and also to those adults who don't have an inherent dislike of fantasy.
Oh, and the best line in the book: "NOT MY DAUGHTER, YOU BITCH!"
Never, ever mess with a mother of seven...