Tests show consistent read in the pictured location, I am testing with a Classic ring.
There has to be contact, occasional read maybe barely off the surface, but not much. and not consistent, i'd say not the best if you were planning to use it for unlock,
will not work with my Otterbox defender case.
added a ruler in the pic so you get an accurate idea of the sweet spot.
Thanks for posting that, should be helpful for others with an S5, and a real shame that it's difficult with the Classic ring!
*edited to show pictures correctly