The acting was pretty bland. I didn't mind because if you are going to this movie after seeing the previews, you could tell that wasn't the focus. It's all about the action, the blood, the gore, and more action, which this movie delivers in spades. Come on, a title like "Ninja Assassin," I hope you don't expect a masterpiece of cinematic genius.
Enough of that, I just wanted to lay it out there that I felt that the action scenes were pretty well choreographed, and kept my interest. However, please note, when did digital blood become the new fad, because this movie had enough to put most 80's slasher flicks to shame. At first it didn't bother me too much, but as the story moved on, it became way too over the top, even for me, who thought 80's slasher flicks didn't have enough blood.
Now on to the story. It is a simple plot that gets complicated because it tries to do too much. I would have liked just a simple revenge plot, which this has, but they had to mix it up with a government agency trying to figure out the mystery behind the ninja assassins. In my opinion this sort of bogged down the momentum.
It wasn't helped by the fact that the acting wasn't that great so the emotion they were trying to convey wasn't portrayed correctly, and the scenes fell flat. Usually when the didn't talk, or not in full sentences it was perfect.
So overall, I thought this movie was worth the rental to watch the cool fights and gore, but everything else was second fiddle and not interesting in the least. This movie gets a solid 2 out of 5 from me. Enjoy it, as long as you know what to expect.