Definitely nothing like the stale Hollywood action genre. Energetic, stylish, and just the perfect length. That said, the cheesiness of the b&w flashbacks and the music choices add a bit of a comical effect, which was probably unintentional. I can't say the characters were particularly interesting, either. The ending won me over, though, and I might just check some more Hong Kong action.
Rated The Departed the same, although I prefer this to Scorsese's remake. IA is a much leaner affair, w/ none of the unnecessary bloat of the Wahlberg and Nicholson characters. Lau's character in IA is much less two-dimensional than his Matt Damon counterpart in TD, but DiCaprio's character is better fleshed out than Tony Leung's. I did like TD's twist of having both men involved with the same woman, but IA is sleek.
Over time this film has become known as the basis for Martin Scorsese's The Departed, but it still stands on its own. Tony Leung is very good, but I think it's Andy Lau who owns the movie. You don't really know which side his character really serves until the end of the movie.