If anyone asked this question a year or so ago, I would have been adamant that the Apple MacBook Air is the last word when it comes to ultra book, not so any more. The simple fact of the matter is that ultra books can do so much more now and Apple has not really made any major changes to the MacBook Air to keep up.
Undoubtedly the one area where the MacBook Air still rules is battery life. The latest MacBook Air 13 running the Intel Haswell micro processor can give almost 12 hours of life on moderate usage. But in almost every other department the MacBook Air falls short. The new Lenovo ThinkPad Yoga 2 supports 3,200 by 1,800 resolution compared to the MacBook Airs measly 1,440 by 900. And the Yoga Pro 2 is a touch screen that can be bent into tablet, albeit a heavy tablet. No MacBook till date has had a touch screen. If compared to the Dell XPS 13, the MacBook fails in almost every comparison, especially in gaming and physics scores.
In short, the MacBook Air still does what it always did very well and it lasts forever, so it seems. But ultra books can do so much more now, and the MacBook Air needs to catch up.