My family got started on the Harry Potter books shortly after the US edition of the second book came out. They were already starting to get a lot of publicity; my mother-in-law was a librarian*, and her library had both books, so she brought the first one home to see what the fuss was about. I spotted it lying on the table and picked it up - and couldn't put it down. And she, my wife and I have bought every book since then as soon as we could after publication. My older daughter started reading them, too, after seeing the first movie. (By now she's read the first six books at least five times each, and I think that as I write this, she's reading Deathly Hallows for the third time....)
I have to admit that by the time this book - the seventh and last one in the series - came out, the questions of who lost, who won, who lived, who died, who married whom, &c, were almost secondary. The thing I most wanted to know was just what, exactly, made Severus Snape tick. We finally get to find out in chapter 33 (of 36); I was right about whom he was really working for (Dumbledore or Riddle), but the reasons behind everything blew my mind.
In the previous book, Half-Blood Prince, it was discovered that Lord Voldemort (once known as Tom Riddle) had split his soul into several parts and embedded most of those parts within objects known as horcruxes; as long as at least one horcrux remains intact, Voldemort cannot be killed. When Harry, Ron and Hermione were last seen, they were planning to drop out of Hogwarts and go looking for both the horcruxes and a means of destroying them. They do, and the fun begins....
If you're one of those who have been turned off by the idea of reading children's books, forget that - this is no longer just a children's series. There's no sex, other than a couple of instances of passionate snogging, but there's quite a bit of violence. Several characters, amongst them a couple whom we've known since the first book, are killed, and others are injured (including one victim of "friendly fire"). Bellatrix Lestrange meets her well-deserved end, at the hands of an unexpected opponent. The final battle between Harry and Voldemort (yes, they do square off at last, and no, I'm not telling who wins - go read the book!) was well done. When Voldemort faced Dumbledore at the end of Order of the Phoenix, he became angry when Dumbledore kept calling him Tom; he seems to like it even less when Harry calls him Riddle.
And speaking of Dumbledore, we get a lot of really interesting background information on him....
Not all is death and violence, of course. A lot of minor characters we haven't seen recently reappear - such as Harry's old Quidditch team (Oliver Wood, Katie Bell, &c), Viktor Krum, the spiders and centaurs from the Enchanted Forest, Dobby and Grawp. Griphook, the goblin who first took Harry to his private vault at Gringotts, plays an important role. We also see the whole Weasley family, including Charlie, Bill and Percy. There are two weddings (one of them offstage), and a baby is born.
A few choice lines:
"If you think I'm going to let six people risk their lives - !"
" - because it's the first time for all of us," said Ron.
Never once had he imagined Dumbledore's childhood or youth; it was as though he had sprung into being as Harry had known him, venerable and silver-haired and old. The idea of a teenage Dumbledore was simply odd, like trying to imagine a stupid Hermione or a friendly Blast-Ended Skrewt.
"NOT MY DAUGHTER, YOU BITCH!"
"OI! There's a war going on here!"
... a herd of galloping desks thundered past, shepherded by a sprinting Professor McGonagall.
I do have a couple of minor complaints, one being that while the Death Eaters are using every unforgivable curse and any other deadly spell they can think of, Harry and his friends are still using stunning spells and trying to disarm their opponents. I also thought the epilogue ("Nineteen Years Later") was a little too cute, though those who are really desperate to know the futures of some characters (Do those two really marry?) will be happy with it.
All in all, it's a good, strong book, one of the best in the series. Rowling does a good job of tying loose ends together in a satisfactory manner. (I'm going to reread the entire series now, in order, using my knowledge of the ending to see what I missed the first time through.)
Update 2209 29 July: Edited to add a couple of points that were in my original (mental) draught, plus a spoiler warning, and to correct some misspelt words.
* I say "was," because she is now a retired librarian.