Harry Potter is a controversial topic within the Church. Is it ok to read? Is it ok to let our kids read it? The truth is, there's no cut and dry answer.
Those on the "no" side argue that Harry Potter is undeniably sinful. It's full of witchcraft and occultic practices, contains some foul language, and really blurs the lines between good and evil. There isn't even a higher power to answer to! Those on the "yes" side argue that the magic in Harry Potter is significantly different from real life, and that there are still great lessons to be learned from it.
In this case, neither side is clearly right or clearly wrong. Those on the "no" side have some excellent points. Anything associated with the occult, such as witchcraft and necromancy, is clearly forbidden by the Bible as a detestable practice.
That being said, those on the "no" side very rarely read the books or watch the movies. They just hear the word "witchcraft" and automatically assume real-world occult. In reality, the magic in Harry Potter really is significantly different from the magic of the real world. The wands, for example, are created using parts of mythical creatures, such as the tail hair of a unicorn, the feather of a phoenix, or the sinews of a dragon. The magic also seems to be genetically encoded, integral to the being of the individual. The wizarding world is completely ignorant of the real world, even to the point where Arthur Weasley has to ask Harry "what exactly is the function of a rubber duck?" The series bears little resemblance to the real world at all.
This is both an advantage and a disadvantage. While it is advantageous in that it is 100% impossible to imitate the wizarding world of Harry Potter, and thus it cannot logically be sinful, it is also a problem, because there is not one mention of God. This isn't surprising, as J.K. Rowling isn't a Christian, and is actually very much anti-Christian, (let us pray that changes), but that does make it extremely inadequate for Christian teaching, unless you know what you're doing.
The foul language is comparatively tame for a modern, popular series. The "big" swear words are, at least to my memory, completely absent, and the ones that remain were big for the period in which the series takes place (the 80s-90s), but have fallen out of common use, to the point where most people don't even consider them foul anymore. This does cause problems, however. Obviously, it isn't ideal for children to hear such language, at least until it has fallen so far out of use as to render it meaningless in our modern world. Similarly, swearing is a contagious habit, so those who are more prone to it should avoid the series. On the other hand, even the KJV contains language which is currently considered profane. The word "piss" features 8 times (including variants such as "pisseth").
The moral ambiguity of the series is also a somewhat valid argument. On the other hand, it must be noted that no one in any story is going to be morally perfect. The only exception is a true story about Jesus. Regardless of the moral flaws of the characters, there are some very clear moral messages sent. Like all good stories, good ultimately triumphs over evil. Greed, lust, pride, envy, and murder are all presented as clear evils, whereas love, loyalty, selflessness, and courage are all seen as virtuous things, which are seen both as tools to defeat the evil, along with being their own reward.
As a whole, there are merits to both avoiding the series and to using it. The key to determining whether or not you should enjoy it, or allow your children to do so, lies in the mind of the individual. Stronger and wiser individuals will be able to both resist any temptation that may present itself and to use it to further the Kingdom of God. Those with weaker minds (which is not meant as an insult) would be far better avoiding it. Let each of us be fully convinced in our own minds whether or not we are permitted to consume any form of entertainment, using sound judgement.