This is probably the best Batman story ever written - it reinvented and renewed the character, made the Dark Knight a more frightening and frighteningly real person and made the graphic novel into a modern art form. Not bad comic book...
Miller's writing here is excellent (unlike the bizarre angry, sweary, trying-so-hard-to-be-gritty-it's-just-funny style he seems to be stuck in post Sin City) and the story moves along excellently. At first, things look a little to 'episodic' to really come together, but the more you read the better it seems to get. The art work itself seems a little odd to start with - it's a lot less 'comic book' like than most - but the style shouldn't put you off as it really suits the atmosphere of the story (something that becomes obvious on the Caped Crusader's first full page appearance).
If you've got no previous knowledge of the Batman, this is not the place to start. Try Miller's also excellent 'Year One' instead. But make sure you DO get round to reading this gem. Just quit before you read 'All Star's Batman and Robin'...