They are just plain fun. In this case it just happens, that they are fun for alot of people. Just give it a try and you will find out. They have an interesting but not too complicated story. The story is told with a single story line (dont you hate jumping between story lines just when its really interesting??) and its a battle between good and bad. What makes it so simple makes it so good. And the well described little details in between give it the right kind of mix to find them very entertaining.
The addiction is forced by the media i guess. I read my first Harry Potter during a vacation in australia, because some other backpacker was reading a "kids book" made me read Part one. During those 4 week i read the first 4 novels (4 just came out end of my trip). Guess it really is an addiction.
But there have been many stories out there, that are much more intriguing than Harry Potter. But also much more complicated. So i am still going to read Harry Potter and have fun doing so (it is kind of like watching tv. turn off your brain and feel good). But i also enjoy the twisted minds of other authors.