The screen/tablet alone weighs 1.6lb, while the whole unit is a very portable 3.34lb, or 3.48lb with the extra GPU option.
Then the comparisons with a MacBook Air are way off the mark: people don't buy them because they're fast – though they're usually fast enough – but because they're extremely light and can last a working day without a charge. Storage is more of an issue which is why Apple knows to charge a premium for it.
This machine may be powerful but it's also heavy and you're paying a lot for a tablet part you might not use so much. Colour me sceptical but I don't see the demand for these devices coming close to that for Apple's fare.
Be interesting to see how good the sales for Apple's even lighter (and more crippled) MacBook have been.