I have been using the Yotaphone for a few hours now and while the sweet spot is good with the new ring it's not exceptional. The One M8 is probably easier to use with the ring as the sweet spot is in an obvious place so easier to find.

I think in a few days I will have mastered the Yotaphone though and I think our next step should be to show the sweet spot on the eink unlock screen when the user is trying to unlock :) That would be awesome..