We have the WII fit and love it. I don't depend on it solely for exercise but we find the games great fun especially like the balance games and we are moving instead of sitting and watching TV. But you have to use it to get any benefit. Just like a treadmill, if you don't use it then you get no benefit from it.
I've had my Wii Fit for 10 days and have used it every day for 30 to 60 minutes per day. I expect some people do purchase a Wii Fit and never use it - just as I once joined a gym and never went. Does that mean I should tell people not to join gyms? That would be silly. Obviously, if you're pretty sure you won't use something, don't buy it - but I've already used my Wii Fit more than I used my gym membership.