Chap,
I just watched Infernal Affairs last night and have to agree with your remark about how strong the plot is, which is quite a change from many HK films I've seen over the years. With the success of this movie in Hong Kong, I hope the filmmakers over there will take note and start a trend where scripts are polished to the degree that Affairs's was prior to production. (Heck, I hope Hollywood follows a similar trend.)
While the script was quite strong, I thought the work of Tony Leung and Andy Lau in the leads were even better. Tony Leung, in particular, is absolutely mesmerizing as the undercover cop whose life and sanity are increasingly jeopardized as the story progresses.
Kudos also go to Alan Mak and Andrew Lau for their excellent direction. The pacing and editing are top-notch. I was on the edge of my seat during several scenes, which doesn't happen often to me during a film.
For anyone who's not familiar with this movie, please note that it's not an action flick but rather, a very good crime drama. If stuff like The Usual Suspects and Reservoir Dogs tickles your fancy, definitely give Infernal Affairs a look.