"Infernal Affairs" was a real event for Hong Kong cinema. Of course, after the strong box office success of the film at home and abroad, it was decided to make a sequel. And at once a sequel and a prequel. Especially since there was quite enough material to create.
The second part is a prequel to the first film. It was a very good one. Consistently good script, acting, soundtrack, visual style, dynamics.
Of course, you can scold it for predictability and far-fetchedness of some scenes. But the prequel is very plot glued to the original. It's very good that, having died in the first film, the characters Anthony Wong and Eric Tsang are played again here (and also perfectly!). Some scenes, by the way, are full of drama and may well shock especially impressionable people. Of course, the original looked fresher and more interesting, but the prequel turned out very decent.
By the way, if you didn't watch the first movie, you may start watching the saga from this one. The main thing is do not dare to prefer it to watching "The Departed", which is much weaker.