It's one of my favourite Joker stories, actually. The plot's good but nothing extraordinary, although the Joker does some disgusting, scary things and Batman's hard-pressed to win in the end.
All that I like, but what I like about it is simply the Joker himself. Brubaker and Mahnke get the character. He's funny! Even as you're recoiling from the horror of his actions, he's funny. That's partly Brubaker's dialogue, but mostly Mahnke's art. He has all kinds of facial expressions, not just his trademark grin. I can't get enough of him. I want Mahnke to draw the Joker for ever (although I'd point out that he's great at giving everyone interesting, distinctive faces, from Jim Gordon to chemical plant workers who are only around for a couple of pages).
"...okay, uncle, you win... I'll kill them some other time..."
The only hiccup caused by Mahnke's scratchy style is that on one page, Bruce Wayne looks at least in his fifties, yet this is the Joker's debut and not long after Year One.
At the same time, though, we're seeing the dark, grimy nightmare through the eyes of Gordon and Batman. This is a good Gordon story too. Realistic urban streets, abandoned industrial plants, etc. There's plenty of atmosphere. The Joker's toxin's effects look disgusting, with grins so wide that it looks as if the victim's flesh is being peeled off their skull. Mind you, Bruce does something weird in deliberately giving himself Joker toxin when you'd think he could have achieved the same effect more safely with make-up.
This, for me, is what the Joker should be. Accept no substitutes.