I got to play around with a Mix at a Xiaomi store. It's an absolutely beautiful phone. The rounded edges of the screen, that I thought would both me, didn't. It fit nicely in my hands, and my fingers didn't wrap around. But, like most, I let my phone lay in my hand with the tips of my fingers pressing against the side. There was no way my fingers were going to accidently press the screen or block the screen. A few things....
1. There's no home button. It's software based, much like iOS accessibility settings
2. There's no intuitive way to interact with such a large screen.
My thumb would only make it halfway up the screen, so to fill in things at the top, I would have to hold the phone with two hands. I tried swiping down to bring the top of the screen down (didn't work), and even held it in landscape, but it didn't automatically switch to landscape. So, I can see why it's called concept. It's a great idea, but there's still some issues to work out.
If the phone were a bit smaller 5.5, it would be awesome! But it's simply too big with not intuitive way to interact with all of the screen.