Ok, so you've seen the two or three nega-reviews below this (or are going to have seen, whatever). Maybe you're wondering, if these noted critics think it so bad, why does it have so many fans? And the simple answer is, because it's good.
It's not good to everybody though, obviously, otherwise the people below wouldn't have negareviewed it. I'm not going to suggest that they just didn't get it, or that
True Art Is Incomprehensible. I agree though with many others that
Fan Dumb has absolutely ruined people's expectations - it's not the second coming of Christ, it's just a pretty good anime. So ignore the
Fan Dumb. I came into it hearing, basically, "This anime is nice", and that was about it - the rest hooks itself.
Everybody's complaining about the
Anachronic Order. They say that it's horribly confusing. That's the
point. And if you don't like it that way, just watch it in Chronological order (everybody is right about that bloody homemade movie though).
But the true reason to watch Haruhi, in my opinion, is the
Genre Busting. Is this a shoujo romance, a shonen action show, a mystery thing, or some slice-of-life? It's all and neither. It's a
story, a story that treats viewers like people, not morons; a story that is only meaningless if you only take it at face value. A story about a baseball game? Well, it must
only about a baseball game! Never mind
Character Development in the actions and reactions of people, what they ask, what they think.
Subverted Tropes abound, the animation is pretty damn high quality, and one of the most
Deadpan Snarker narrators ever to grace the tiny tiny screen (I don't think I even remember an anime with a single consistent narrator).
So if you're looking for the same thing you've seen before a thousand bloody times in whatever bloody genres you've already seen and know off by heart, go ahead. Look elsewhere. But if you're looking for a unique story that's not afraid to be serious or just to be hilarious whenever appropriate, if you're looking for something that isn't just shoe-horned into the thousand genres of anime, if you're looking for a story that treats you like you know how to use your brain, try the Melancholy of Haruhi Suzumiya. Just don't expect the single greatest work of fiction ever, and you should be fine.