I finished the last Harry Potter last night. I am sure I'm not saying anything new here when I say that it's a fitting finale to the series. Now I have an urge to reread all seven books to pick up on everything that I missed the first one or two times.
The best "children's" books - and I don't even want to call them that because it's a limiting label - are the ones you have on your bookshelves when you're well into adulthood. I for one have the "Wrinkle in Time" trilogy and the Narnia books from my youth and the "His Dark Material" trilogy [Daniel Craig as Lord Asriel - nice bit of casting in the upcoming movie version of "The Golden Compass"] and Harry Potter from the recent past. And as I'm typing, it hits me that these books deal with alternate worlds and realities inhabited by very human characters. All are variations on the classic hero's quest construct as popularized by Joseph Campbell.
Reading is an immersive experience. We expect to take something away, be it raw knowledge, escape or a different perspective. Phrases stay with you. Visuals inhabit your waking life. You reference the book in everyday conversation. I'm not expecting to call anyone a Muggle anytime soon, but I do love the "Voldemort Votes Republican" bumper sticker I saw on a beater of a car recently.