Usually my tastes align pretty much with the score than MetaCritic (and RottenTomatoes) gives for a particular movie. This is one of the few exceptions to that rule. I disclaimer: I am Japanese and live in Japan, and have a sweet tooth for old school Japanese dramas (not just the masters like Kurosawa and Juzo Itami) and styles that some people might not have the taste for. Note that in Japan this movie is getting good reviews -- but that's probably due to the Japanese director and the Japanese actors in it. However, there is a lot of Korean influence (according to the credits) in it as well.
If I was to pick anything wrong with the movie, the second act, which builds up the twist to the third act and the obvious final battle, was a little too obvious to me, and those familiar with this type of drama, even Clint Eastwood's old masterpiece, "Unforgiven", will probably recognize the device right away.
The combat scenes were very well done and while choreographed, there was a minimal of "you must suspend your disbelief" type Matrix-like line-and-wire acrobatics (there was some though).
Finally, the movie doesn't wimp out with a Hollywood style "Happy Ending." I'm not going to tell you if it's a good ending or not... I'm just saying that they didn't choose the ending that would have marketers would have preferred. Good for them.
