I wish people would stop calling it emulation.  They are using binary translation of the x86 code to native ARM code and caching it.  This is not new technology, it was used in the past on the early DEC Alpha machines running NT 4.0.   You should really use one before saying its really slow, it keeps getting faster the more you run the x86 apps as the JIT
optimizes the translation more.   

Don't fall for the marketing hype.  An arm chip is still a long, long way from being a suitable replacement for an x86 processor in any computer used for more than email and movie watching.

The only use case is where battery life is your paramount concern.  Also really don't believe the hype about Apple's processors.  They strip all kinds of stuff out and optimize the chip for single thread performance.  That can work if you run an OS which is incapable of multitasking.  Try running a modern version of Android with one of them and the system would slow to a crawl before the chip melts its way out of the phone case.

Given the pace at which Qualcomm chips are getting faster (much higher than for Intel ultrabooks), there is no doubt that ARM will soon outperform Intel at equal and even lower TDP. So in 2-3 years we'll have chips that are natevely faster than Intel quad core U chips (general windows use, Edge based on chromium, office etc.), that is to say a speed like what the ARM chip in the ipad pro already has, around 18000 in multi-core Geekbench, while showing maybe dual core like performance in Emulation (that is around 8000-9000 with the same benchmark). This could be a big hit in  the market if priced reasonably, contrary to what OEMs have done so far (selling at extremely high margins).
We probably need these chips to fall in the hands of Chinese OEMs to get prices down...
One way or another WOA is likely to be a big part of the ultrabook future in a 5 year horizon.