Been living in Japan for about 12 years now and have watched how Halloween went from practically nothing to being celebrated throughout Japan. As the media pick up on festivities, more and more folks join in on the halloween shenanigans as each year passes.
In Tokyo alone, just a walk down Shibuya, Shinjuku, Ikebukuro or just the local shotengai, one will see a load of Halloween decorative bits n pieces here n there. There are also many notices for Halloween parties and events too.
However, while more and more kids are going out these days to trick or treat and folks are getting dressed up, its still more of a "decoration" rather than a "celebration".
I avoid going out in armor on Halloween as I don't really want to be associated with the drunk noisy gaijin that go around stripping their clothes off on the Yamanote line.