I think when the Potter books were becoming a phenomenon a number of Christians were worried, concerned about these books and the insight into magic and witchcraft that lies within. Some of my friends were among those concerned so I must be careful what I say. Anyway suffice to say, I don't think the population of warlocks and witches in the UK has gone up significantly despite the millions sold.
I read the first one a year or so ago and now I'm at number 3 (having picked this one up for £1 somewhere in Mid-Wales) and these are just cracking reads. Rowling really does tell children a good story. I think they could be a bit scary so caution is advised with werewolves and monsters. But that applies to Narnia and Middle-Earth as much as Hogwarts. There's humour and plot, wonderful characters that fire up the imagination and you root for the hero. There's so much to commend. I thoroughly enjoyed my third trip to Hogwarts and look forward to a return soon.