The preliminary Japanese data tends to suggest that Wii Fit is not a fad. It's not unreasonable to extrapolate its success there to NA and Others. It's certainly not everyone's cup of tea, but you can say the same thing about every other game too.
I don't have it yet, it'll be arriving on Thursday or Friday, but it's my most eagerly awaited game this year. I consider myself both a hardcore and casual gamer. As far as I'm concerned, the terms aren't mutually exclusive.