It's fun and absurd and complicated and fulfills the inner need to overanalyze that a lot of geeky people like myself have. Me and a bunch of friends in college were reading it as it came out and speculating wildly on what would happen next, especially about motives, and especially around Azkaban. But it was just a popular fanfic with an absurd hilarious premise that we liked (and that would occasionally lead to me oversleeping and missing the first half hour of my astrobiology class). Another work of comparable interest that had quite a similar effect on several of my friends around the same time was Homestuck. Nowdays we have nearly completely lost interest. Partially because we have learned of the existence of people who take it too seriously / have changed their life-pattern after reading it which adds an ugh effect and partially because it's become rather overdone / takes itself too seriously. My interest in it that remains has changed from fun to morbid fascination.

One more thing-- it's got is tremendous emotional range and intensity.