There's just something about this film that's perfectly unique. It gives off a kind of gentle, calculated atmosphere, while depicting haunting and simple madness. Robin Williams gives a brilliant performance thanks to the great writing, which is augmented by the top-notch cinematography and perfect soundtrack.
Really, though, what shines here is the story. It's a look at a seriously deranged person, which is fascinating in its subtlety, but climaxes up to a brilliant and satisfying ending (which sounds really dirty, in retrospect). Plus, it's got more than one explanation, which is always good.
In a sentence:
Not for everyone due to its calm nature, but subtly brilliant with great and controlled production values.