These are my thoughts after re-watching Aau Chin on a VCD recently bought at HK$15 (definitely NOT a pirated version). I still found it quite enjoyable.
The "setup", while not brilliant, is carefully done, with attention to details. No doubt the audience will find the total incompetence of the villiant "Baudy" quite unbelievable. But he only serves as something on which the duel between the two heroes is built.
Here's is where this film differs from many others in this genre (most notably John Woo's), in which the two heroes usually start off in sharp confrontations, although they may end up appreciating each other. Not here. Right from the beginning, Lau Ching-wan and Andy Lau give you the impression that they are engaged in no more than a friendly game of chess. I, for one, like this arrangement, as a refreshing change.
Lau Ching-wan delivers his usual easy style that is well liked by his audience. He is the mouse in this cat-and-mouse game, a dignified mouse, for that matter. Andy Lau is the cool cat, actually too cool. Hong Kong movie stars who are also Canto pop stars (and that accounts for some 90% of them) often have to watch the role they play in movies to ensure that they wouldn't tarnish their image as a singer. Andy Lau, however, is so popular that I don't think he needs that caution. He just like to look cool in this movie. Didn't hurt either, as it won him his first Hong Kong Oscar.
Finally, while this is a good movie from director Johnny To, my top favorite of his is The Mission, in which style is king.