Getting back to the topic...
Personally, I find the HP books very engaging. I couldn't stop until I finsihed the HBP in one sitting, it was really enthralling.
But, I would not want a very young person to be reading that book. For me, it has the potential to encourage young people into cultic behavior. It's already happening, right? Waiting in line for three days to get a copy of the book and all that...
I can't help comparing it to Tolkien's LOTR and Lewis' Narnia where we know these are different worlds and the line between fantasy and reality is very sharply drawn.
In the HP series, magic occurs in this world of ours. The line is not clearly drawn. Kids with a very vivid imagination will want to try and prove that they are wizrds and witches.
I just feel that grown-ups should discuss the book with the little ones reading it, just to make sure that they are reading it as a fantasy, and are not tempted to act on it.