I have a Wii, so hopefully I can help a bit.
First, the main game system comes with sports games like bowling and golf. These won't be on the same level as a regular cardio workout. I think it is great if you are using it to replace otherwise sedentary things (like if you would normally watch TV, play Wii instead).
Also, the Wii Fit is an extra you can buy that costs more than a game - it is almost an additional system. This is the thing you see people standing on. I got one for Christmas and it is pretty amazing. I still don't use it as a replacement for cardio, but I can get a quick small workout in on it for a little calorie burn. It comes with a bunch of games I personally find really fun, so we haven't bought any additional games with it. It also has strength training and yoga components that I have personally found plenty challenging to help develop muscle (for me). There are also aspects that help you stay motivated - weight/BMI graphs, "points" for the number of minutes you work out (and you can enter minutes you spend doing workouts elsewhere), etc. All very fun.
Overall I recommend getting the Wii and investing in a Wii Fit before you buy any additional games. If you're like me, once you get the Fit, you won't want to play any other games. :)
I hope this helps answer your questions!
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