I don't think WiiFit is going to take off in either NA or PAL regions. It will sell well, but it won't sustain sales like it does in Japan for a few reasons:
1. The Japanese love that sort of stuff. Quirky non-games are all the rage over there and the Japanese love their fads.
2. WiiFit is targeted toward an entirely casual audience, which is fine. The problem is that the Wii is still supply constrained. The people who will buy this won't be able to find a Wii to actually play it.
3. Those same people don't stand in line to buy a video game console.
4. $350+ is a large buy-in for a pseudo workout machine/rig/thingie.
5. The typical gaming audience doesn't give two shits about this game and won't buy it in significant numbers unless it's insanely fun (which, by most reports, it isn't).
I think some hype will build up around WiiFit and it will sell quite well for a bit and then slowly die off as people realize that finding one of those little white boxes is damned near impossible. Once the hype dies down and the Wii is available, it will be too late.
I could be wrong but I don't see this "game" doing that well in the west.
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