The tale is epic and has all the characteristics of a great read. Whereas I do watch the films now and again, I pick up the entire series and read them more often than most. Especially when the local libraries aren't receiving new releases often enough to keep up.
We have a hero that doesn't really want the job, and has not the skills to accomplish it at the outset. We have a team that will follow him to hell. We have many mentors who realize the hero needs a great deal of help. We have many interesting antagonists. And Quidditch, a game never before conceived.
Finally, the HP World is self-contained and sustaining; even though we know it is fictional, the laws of the magical universe hang together as well as real world physics does. One can deduce and extrapolate from the data presented. The relationships are admirably maintained by JKR, so we can predict where they could go.
This is just a short answer, but these are the simplest highlights. A full treatment would be novel length.