Just finished watching the entire Infernal Affairs trilogy within a week on my spare time. I've seen IA many times but have never quite had the experience of watching them within a close time frame in viewing the darn thing as a whole series.
My two cents on Infernal Affairs and The Departed (which I've dug up one time many moons back but forgot to comment on): it's inevitable that The Departed as an American adaptation of Infernal Affairs is compared down to every nuance. That's understandable, but as similar they are, they're both completely different movies as far as I'm concerned (and excellent ones at that) and they both can't be compared. Scorsese makes The Departed his own through more defined character development dialogue and a more deliberate pace whereas Infernal Affairs is simply more psychological and more tightly paced.
Infernal Affairs I - 9.5/10 - For comparison's sake, I'm biased but I prefer this over The Departed. Better suspense and better theme. Every moment seems to have its purpose, even the smallest of details do, and they all sneak up on you for a shocking but beautiful finale. The minimalist dialogue suffices very well and the film doesn't drag as long as The Departed does in general. I'd take the Tony Leung/Andy Lau pairing over Leonardo di Caprio/Matt Damon duo as well, the latter having to be over-the-top, Leo especially. Tony Leung can skip the theatrics and look equally as credible as a cop. As flexible as Andy Lau is with other roles, I think he's a so-so actor in my book who has had his moments, but does a more than adequate job here.
Infernal Affairs II - 8/10 - To be quite frank, I have trouble rating this one. IA 2 is a prequel to IA 1 but it's also not a prequel in the truest sense. First of all, there isn't that much revealed here in the prequel that makes you view the first IA any different, as one would expect from prequels. There are a few not-so-subtle coincidences that connect the two movies together, but in all, it's a standalone story. Another noticeable difference from IA 1 and IA 2 is the structure and pacing of IA 2. The pace is a slow burn, no flashy sequences to speak of. The story smacks of something out of The Godfather and in fact there's a killing sequence that's somewhat similar to the famous baptism killing sequence found in the first Godfather. You'll look at Infernal Affairs 2 and be puzzled as to how this is connected at all to the first film. That's where this movie doesn't succeed. What it does succeed in is it delivers an effective start-to-finish standalone story. No Tony Leung/Andy Lau pairing here as it's all flashbacks, but the two supporting actors from the first film (the triad boss and superintendent) really make up for it, namely the friendship between the two before it reaches its "point of no return" and turns ugly. IA 2 makes you feel sympathetic for the triad boss, something you wouldn't have expected from viewing the first. The subtleties take time to digest but in all, a worthy watch. (FYI, the backstory elements that were in The Departed but weren't in Infernal Affairs are derived from IA 2.)
Infernal Affairs III - 8/10 - This is more par for the course in terms of styling and pace with IA although it blends with the slow burn of IA 2 at times. This is a true sequel with a lot of flashbacks: post-IA 1 Andy Lau mixed in with pre-IA 1/post IA 2 Tony Leung and as the movie progresses, that past/present line is blurred, which I think is fantastic. Where it excels in that IA 2 didn't is that it makes you view IA 1 differently in that it adds backstory that complements well with the first film. Of course, you'd have to view IA 2 as well to understand the significance of certain events, especially the ending where it really hits home. Andy Lau is a bit more demonstrative for the roles that he usually plays, but it works here. This film out of the three gives more emphasis on the balance between heaven and hell on earth, interestingly enough.
In short, I'd only watch Infernal Affairs II and III if only you're a curious fan of the first. The first movie is highly recommended to all if you haven't seen it, especially if you've seen The Departed before.