I suppose that sort of passionate reaction is key. It's rather hard to feel an emotion that strong about something you don't care about so job well done. But isn't it better to leave them wanting more but offer some sort of closure at the same time?
I mean most beloved classics (at least not any that I can think of) don't end in cliffhangers, even ones with follow ups. They usually are wrapped up neatly at the end. And, while a happy ending isn't always a necessary to make for a good book, at least those that end neatly, with no holes in the story, make for a content reader.
Do you agree? Do you find value in cliffhangers?