However, it also has a lot the Air lacks, including an optical drive, ethernet networking, three USB ports and a built-in cellular modem. You pay for it all, mind you: this little cheese slicer will set you back a staggering $2,500 in its cheapest, most low-end incarnation.

Though I'll readily admit I haven't used the X300, so long as the damn thing actually works properly, its nature is clear enough. If you like the Air but want a computer rather than bling, this is probably what you're really after.
I'd wait for the new MacBook Pros before making up your mind, though.