Wii Fit

This month will see the release of Grand Theft Auto IV. It's variously been described as the gaming event of the year / the defining moment of this console cycle / the pinnacle of human achievement including the discovery of fire, walking on the moon and the invention of the Oyster card. It'll sell millions of copies and more than a few consoles.

But while the launch will be covered in the mainstream media to some extent, you won't see Lorraine Kelly giving it a go on GMTV. Fern and Philip won't be discussing the merits of the new cover system and procedural animations, and Richard and Judy won't marvel at how the motorbikes can do backflips.

Wii Fit, on the other hand, will be everywhere. It's launching on 25th April, four days before GTA IV, and it's going eat column inches, take up all the airtime and fly off the shelves. Will it sell more copies than GTA? That will partly depend on whether Nintendo can churn out balance boards quickly enough to meet demand. Two things, however, are certain: Wii Fit will be huge, and it deserves to be.

For GBP 69.99 / EUR 89.99 you get a copy of the game and a balance board peripheral, neatly packaged in one of those pristine white boxes Nintendo does so well. They even throw in the four AA batteries required for operation. The board has a real heft to it. This is no cheapo bit of plastic (yes, Mr Wii Zapper, that means you), but a serious piece of machinery.

That fact becomes even more evident once you boot up the disc and set about creating your Wii Fit profile. You can register as any of the Miis stored on your console, create a new one or play as a guest. Then it's time to take a Body Test. After entering your birthdate and height, the balance board measures your weight and calculates your Body Mass Index, telling you whether you're Underweight, Ideal, Overweight or Obese.

Your actual weight isn't displayed on-screen unless you hit the specific button, so no one else in the room will know what it is unless you want them to. You can even set a password so other console users can't peek at your weight or Body Test records when you're not around.

Once your BMI has been recorded, you can take a series of balance tests. These involve things like shifting your weight left or right on the board so you stay within certain markers on the screen, or trying to remain steady while standing on one leg. At the end of all this your BMI, age and balance test results are combined to calculate your Wii Fit age.

It's hard to tell just how scientific all this is, but some wild variations in results suggest it shouldn't be taken too seriously. The first time I took the Body Test, I was given a Wii Fit age of 46; the next day I had apparently lost four pounds and 20 years. All that really changed was I chose the "light" rather than "heavy" option when asked to describe my clothing and was better at the second day's selection of balance tests. Friends had similar experiences, which suggests these two variables might have an overly significant effect on the outcome of the Body Test. (Not nearly as much of an effect as alcohol, though; a bottle of wine and a pint of Cheeky Vimto can add half-centuries.)