Not as bad as it could have been. US films about ninjas (and samurai for that matter) tend be quite cheap and cheesy. Ninja Assassin doesn't fully escape that faith, but at least there are some kick-ass action scenes, which counts for something. That's pretty much the only reason to watch this film though.
The film starts off quite promising. A ninja assassination (hah) that is rather bloody and ruthless. Heads fly, bloods spurts and iron flies through the air. But then the plot starts and things start to slip real fast. The performances are weak, the characters are drab and the drama is pretty lame.
The entire middle part of the film is pretty much negligible. It just runs through the motions and spends a lot of time setting up the finale. Luckily it ends with some fine action scenes. The editing and camera work may not be up to par, but at least it's graphic and over-the-top. Decent, but not great.