This is an important work primarily in that it helped to redefine Batman and The Joker into their more modern and complex characterizations. The story is an origin story for the Joker by two of the most important people to ever work in comics.
The writing is good, as of course is the art, but the quality of the story is only as good as the typical work by Moore and Bolland for the British comic 2000 AD. I was raised on 2000 AD and know that Moore and Bolland were churning out this sort of quality on a regular basis. The Joker's origin story is probably not sufficiently traumatic enough for the transformation to that character. The Joker has a personality more extreme and fascinating than Batman's and the story is not quite up to explaining it.
Good but most important historically for the evolution of the Batman story.