I don't think that there's anything wrong with the Harry Potter books. The arguments I hear about them deal with the witchcraft aspect, and how certain [Christian] parents don't want their kids reading that kind of material. Personally, I believe that's going a little overboard. The books deal with witchcraft, sure, but they don't promote it, or devil worship, or anything else of that nature. I don't see them as a bad influence at all, in fact, quite the contrary. They illustrate good deeds and strong friendships. They (to some extent) show the troubles and trials of adolescence, and therefore kids can relate. There's no foul language, sex, or extreme gore.
I think that if any of the parents who were against the books actually bothered to pick one up and read it (with an open mind), they would see the positive aspects, and how the "witchcraft aspect is a bad influence" arguement is really kind of a moot point.
Besides, they're just fun, fantasy books. And at least they're getting kids to read.