For children's books (and even for adult books), they are wonderfully well-written and well-crafted. In fact, I think that J.K Rowlings has really reshaped the whole young adult book market. When I was growing up, the most popular children's books were the Baby Sitter's Club series. (Talk about bad children's literature). Instead of only having that type of garbage to read, today's eight and nine year olds get to read books with well-developed characters and suspenseful plotlines.. all because of the Harry Potter phenomenon.
How old are you?
I guess you never read any classics. You must never have read any books by Robert Louis Stevenson or even Louisa May Alcott.
I guess kids now just read pabulum, so the Potter books compare more or less equally to what passes for juvenile literature today.
I grew up reading actual literature, so I see the Potter books differently - as shallow, two dimensional comic book style tripe with not a hint of any morality, upliftment, real mystery, empathetic characters, believable three dimensional (or even two) characters, or anything else that makes real literature literature.
To say that the Potter books have well developed characters is like saying that Britney Spears intelligence has an impact on world affairs.