The plot is very interesting, and there's tons of tension! Definitely keeps you on your toes as the pressure mounts for the main characters, especially in the undercover scenes. There wasn't really a point where I thought anything in the main storyline didn't make sense; the motivations of the characters are as clear as they should be, and the emotional weight of the story is perfectly translated every time. The immense stress that they're under is very compelling, and the inner struggle of the characters is fantastically depicted.
That being said, I thought some of the side characters and side plots were superfluous, not developed enough, or just straight up didn't make sense. Inspector Lau's fiancé's novel is a bit on the nose, and Yan's relationship with his psychiatrist came out of nowhere.
The acting was pretty good, especially for the two main characters and their respective handles. Some side characters were also pretty cool, I really loved Chapman To in particular.
The cinematography had some highs for sure, but also some lows. There were some shot transitions that looked like someone just pulled up WMM.
Overall, it's a cool film. I decided to watch it because it's highly rated and from Hong Kong. I'd previously watched Chungking Express and In the Mood for Love and absolutely loved them, but Infernal Affairs simply doesn't compare. It's OK, and definitely worth a watch, but it didn't blow me away.
4/5 for me when I saw it in 2002. It was so good Scorsese made an English language version and won Oscars. I think I view it in the context of other HK gangster films, which are numerous. There are definitely some tropes I think I "fall" for (bad guys having morales, bad guys with personalities like good guys), but overall I found it very original.
The Scorsese movie would be The Departed, right? I might give it a watch, just to see how it compares. Wikipedia says the script for Infernal Affairs was inspired by Face/Off, by John Woo. It's funny how that goes, American film inspires HK film which inspires another American film.
The double infiltration scenario was really cool indeed! One-dimensional bad-guys can be rather predictable and boring, it really is great when the antagonist has some good-guy traits. :D