Like Snakes on a Plane, "Alien vs Ninja" is one of those movies you get just for the title. the execution will never match what the title inspires...
A low budget Japanese action flick... the film is basically " Predator" except set in feudal Japan and with Ninjas instead of commandos... and without the production value or cool alien predator. In this case its a bit closer to Godzilla rubber suit bad guy hunting them down rather than a cool high tech alien.... :-P
As this is a Japanese B-movie.. you shouldn't be surprised that the alien also happens to be a pervert alien, taking the opportunity to sexually harass the female ninja in the film when possible. (its called 'Fan Service'... look it up on wikipedia).
The writing is very tongue in cheek and over the top in many places. The action is quite good considering the low budget... and the CG is not that bad either, when it is used....
Actually the biggest downfall of the film is that the alien looks too much like a guy in a cheap rubber suit (who's a pervert). :-P
The special effects of people getting a chopped by swords or ripped apart by the alien and whatnot are done pretty well too... maybe they should have made the alien look more like a human and it would have come out better... or played the ' Jaws' strategy and only actually show the full alien at the very end...
Anyway, as an extremely low-expectations cheesy Japanese B-movie, its entertaining. Check it out if you get a chance - 6/10.
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