Look at it this way: The closest in terms of portability and price is not the Asus or the Vaio Z, but the Vaio X. And the Vaio X sucks badly when compared to the Air: keyboard, CPU, GPU and screen are much worse.
The C2D gives you a great thermal design, and with respect to speed it is in the same league as last year's MBPs. Thus, you will be fine for a portable system.
If you are a portable gamer then you should look for the Alienware thingies, but if you want a system for writing, internet surfing, presenting, light programming and so on: this is it. The question is not if there are faster systems out there. The 11.6 Air might just be the best overall package. It is the only ultraportable with MacOS, after all...
Well, technically even the universe is a computer, in the sense that it can do computations. However, the iPad is no PC, and this is probably the point. For me, the iPad is a glorified media player. Yes, you can compose an eMail on it, and in a pinch, you can even take notes. But for productivity? Not this year.