Here's a way of looking at it:some time back Denise Crosby took part in
a documentary that followed some really hard core Trekkers around.They
called the film:TREKKERS(or was it TREKKIES)?Any case it's pretty famous
(or if you want infamous)in Trek circles.
My point here is that an otaku would be an equivalent of a die hard
Trekkie/Trekker here.Someone who gives up all their free time in pursuit
of Japanese Pop Culture.
However both can give more moderate fans a bad name so you do have to be careful.