I tried a Tempered Glass one on my M8. It was ok. It was $18.00 but that phone died.
I used a conventional screen protector on my replaced M8 and my S5.
The Tempered Glass one was VERY slippery. I swear if you put the phone on a glass table face down it would float off like a hockey puck.
I didn't like how it cleaned. It cleaned like plexi-glass and just didn't feel crystal clear.
They are thick. The corners will not curl on you because it's stiff.
I think I prefer the standard type. I used these on both the S5 and M8.
Some might complain they don't right to the very edge. But that makes it easier to install.
I managed to get it on absolutely perfect. Less visible than plexi-glass. Feels more like stock glass. And cleans more like stock glass.
I will say though the plexi-glass one seemed to less oil from your fingers. Same as stock glass.
This is the plexi-glass one I used.