I started reading them because everyone was making such a fuss about them. I had to see what all of it was about. Low and behold, I was hooked from the first book. Book 4 is my favorite and I can't wait to see the rest of the movies. Somehow, I managed to get my boyfriend hooked on them, even though he refuses to admit it
To answer the question...I like the books because they are simple enough for a child to read and understand, but they are deep enough for an adult to enjoy. I find myself wishing I could go to Hogwarts and go to Potions class. When I have kids, I definately plan on reading the books to them.