I've read all of the Harry Potter books and I loved them. They were fantastic. I can't wait for the movie premier next month and I've been on pins and needles in anticipation of the next book. My interest was kindled when I began reading it to my kids. Unfortunately, at ages 5 & 6 (at the time) they would inevitably fall asleep long before I was ready to put the book down. I finished the entire series on my own before I had finished that first book with them.
Just as an aside: witchcraft has nothing to do with devil worship or magic. True practitioners of the wicken tradition, from what I know of it, are worshippers of mother earth. Harry Potter is a fictional "magic" character. Rowlins chose to call him a sorcerer in order to capitalize on all of the, really erroneous but entertaining, myths we've been fed about witchcraft. If you're worried that your kids may be somehow corrupted by the book, read it with them in order to help them interpret it. This might be a good time to investigate with your child exactly what the Wicken tradition is and what Pagan rituals involve - a good lesson in rejecting mass interpretation and looking for the truth beyond what's given. Additionally, terms associated with "witchcraft" will lose their mystery and therefore their power (an idea explored in the books - a name gaining power simply because of the fear surrounding it) to scare your kids.
Withcraft/sorcery, phoney baloney, whatever you chose to call it. It's not that serious. I look at the series as one about human beings with all of our good, bad, and ugly faces. We, unfortunately, cannot do magic so my kids naturally emulate the protagonist (Harry, the good guy) in the ways they can: by exhibiting humility, being loyal to their friends, and showing courage in the face of danger (which could simply mean speaking their mind on the playground).
Wonderful book. Lighten up you silly muggles!