It's FAR better then most say.
Story: I'll be frank, I wasn't too keen on MD1's story due do it's semi-bland simplicity. Luckily, Chunsoft got the right idea and made a plot for the sequel that is surprisingly deep and well thought out. It starts as a standard rescue-oops, I mean EXPLORATION team, but soon becomes much more.
Graphics: The graphics here are pretty slick. Each and every Pokemon is rendered in very nice 2D images. The enviroment graphics are no slouches either.
Gameplay: If you've played a Mystery Dungeon-esque game before, you know what to expect. If not, all major actions such as moving and attacking are turnbased for you and your enemies. The Dungeons you go through are randomly generated, and always provide a new experience. Still, some players may be put off by the fact that the normally-light difficulty spikes suddenly in the second-to-last main story dungeon and onwards.
Still, Pokemon Mystery Dungeon is an overall fun game that no one should miss out on.