Honestly, I really enjoyed this film.  It tells the story of Riazo, played by Korean pop star Rain, an orphan who is brought in by the Black Sand Ninja clan, from his training from his eventual defection from his clan.  Lots of flashbacks and training sequences smattered about between the intense action sequences, I just really liked the movie.

Within the first minute, the tone is set as the top of a triad gang member’s is lopped off with his body spurting blood on his friends.  Repeat for the next 99 minutes and go home happy.

I also like how the movie follows the ninja henchman paradox.  This theory states that the toughness of a ninja is inversely proportional to the number of ninja present.  While one single ninja represents an unstoppable killing machine, in large groups they tend to die rather easily.  We also see the impact of this theory in that as the ninja henchmen are killed off, the individual ninja get tougher and tougher.  So a small piece of financial advice to you, a ninja bodyguard is a better investment than an army of ninja henchmen.  Just an FYI.

So, in summary, see Ninja Assassin.  It is worth it.  It is gruesome and fun, everything you would want it to be.