That one is lost on me, as I have a few missing bits that make it all but impossible for me to walk without paying attention to the walking full time 
I don’t dispute that a keyboard is great. But as someone who gets easily gets 120+ wpm on a full size 104-key keyboard, any typing arrangement on a cellphone is going to be a compromise anyway.
The best solution I’ve found so far is that Swype-like keyboard thing, which is surprisingly quick and accurate to input plain English, even when you’re shaken like a bag of popcorn on the bus or something. And the Swype-ish soft keyboard also has left and right keys - if not up and down. For anything else that requires more involved or faster typing and requires an attachment to the cellphone, I sit down an pull out one of these:
Your little device sits somewhere in the middle in terms of practicality for me: it makes the device too unwieldy for casual use and it doesn’t seem to provide fast enough input to make up for the extra heft.
But mostly it’s a fantastic creation and I love it just because it’s so damn neat. And it IS small enough that I would want to carry one around with me all the time just for the coolness factor Fairphone would be well-advised to sell an accessory like this. I bet it would fly off the shelves.