Mark Shea has a phenomenal post (replete with spoilers) on Harry Potter. It delves into several of the most prevalent arguments against the series and rebuts them, I think, very well. (I'm sure he'll be relieved to know that I approve of his writing.) He also has a lengthy comment from a reader that gets to the heart of the problems we have with Dumbledore in the last book.
I won't post it here, but when you go, be aware that there are spoilers, so if you haven't read the last book, don't say I didn't warn ya.
Read it here.