There are times when I'm standing on the tube wanting to type but the 6S Plus is too big to comfortably use one handed while holding on with the other.
The one handed keyboard in iOS 11 aims to solve that but it still feels incredibly awkward to hold the phone while typing one handed. The weight of the phone makes it feel like I'm constantly going to drop it as you still don't have a solid grip. Yes, everything is within reach of a single thumb but it's pretty precarious.
Interestingly, what does feel quite natural is supporting the phone with my left hand while using the right-aligned one-handed keyboard with just my right thumb. It's obviously not the way it's supposed to be used - cue the cries of "you're holding it wrong!" but it feels surprisingly good.
I’ve got relatively small hands so the reduced distance my thumb has to travel makes for quite a nice experience (at least for me) even though the tap targets are significantly reduced.