The fourth book in the Harry Potter series has a pretty slow and meandering start that lasts for about 3/4 of the book. Maybe I was a little burned out on Harry by the time I started it. I also remembered the movie pretty well, and maybe that made it less exciting. I do think the plot moves more slowly than in the others and it's not even really clear what the plot is, because the connections between events are intentionally unclear until the end.
However, the end was so suspenseful and exciting that it made up for the rest. Even though I knew what was going to happen, I was riveted. It also set up the fifth book in a way that the movie didn't.
So I guess I'm a Harry Potter convert. I resisted for a long time. But after I finished Goblet of Fire, I had that familiar bittersweet feeling of missing the characters as if they were real friends that I'd lost contact with. I can reread the books and see the movies, especially the coming sixth and seventh, but there won't be any new adventures. So why is that feeling bittersweet, instead of just bitter? I think it's because knowing the characters was a good experience, one I wouldn't give up. And while the Harry Potter books are great adventure stories, there's also a lot to learn from them, and not just about really evil evil vs. regular good, but about friendship and growing up.
And here's my favorite Dumbledore quote, from the movie because the book has apparently been swallowed by the black hole that is the kids' library: "Dark and difficult times lie ahead. Soon we must all face the choice between what is right and what is easy."