Lenovo's X300 has been touted as a MacBook Air rival by many, too. It can't be denied that it's faster (albeit running Windows, not OS X) and in its own way almost as attractively designed. Pricing is fairly similar too, and the X300 integrates a DVD drive and has a higher resolution screen. That said, I'm disinclined to think anyone would look at the two systems and seriously debate between them - the X300 is a ThinkPad, designed for business users not consumers.
The Samsung X360 is a more obvious alternative, designed as a direct rival to the Air. It has the same 13.3in form factor and although its CPU is slower at 1.2GHz the X360 is still a nippy enough system, comes in SSD and non-SSD flavours and has significantly more connectivity options than the Air.
However, while it may offer a similarly specced system at a better price - some £200-odd less for the SSD system - with a better bundle (Samsung includes an external disc drive as standard), it simply isn't as desirable. Nobody seriously considering a MacBook Air would look at an X360 and think: "yeah, actually I'll have that instead."
In fact, if you ask me, the only laptop out there that can really compete with the MacBook Air is the Z-series. It's not as attractive, it's not any less expensive but it is a fantastic machine and it actually has features. Everything else aside, however, the Vaio Z still isn't a Mac, which is pretty much a deal breaker in and of itself. Riyad and I have both stated before that we're firm believers in Mac OS' superiority over Windows, so the Air has an instant advantage by virtue of its operating system.
The MacBook Air is incredibly beautiful both simply to look at and to use. Apple has always been ahead of the curve with its industrial design, but the Air truly is something special. The brushed aluminium chassis may add a price premium but honestly, you only have to use a MacBook Air to stat thinking it's a premium worth paying.
In figures it's a slender system, tapering from 19.4mm to 40mm closed, but in the hand, or on the lap it feels even thinner. The aluminium construction pushes the Air to 1.36kg and although that only makes it 60g heavier than a Sony Vaio TT, for some reason it feels heavier, but not enough to stop you carrying it around with you all day, every day.
In an ultra-portable, though, I'd argue that footprint matters more than thickness. As such, the MacBook Air isn't the best example as in that respect it isn't any smaller than a 13.3in MacBook Pro - although the latter of course can't fit in a manilla envelope, making it all but useless for expensive intra-office messaging.