I have an Otterbox Commuter for when at work but generally take it off once home or when going out to some none work related activity. Not a big fan of cases really but the Otterbox does afford some protection should it be dropped. The power button isn't covered but the 'hole' in the case does make operating it quite a bit easier as do find myself having to look for said button when phone is bare. Although I don't like the position of the power button and find it fiddly to operate, I've never turned the phone on by accident in my pocket.
On the protection front I did buy and fit a Spigen SGP GLAS.tr SLIM. It's a sheet of glass to stick on top of errr.... the HTC Ones glass screen

It's maybe not quite 1mm thick and pretty easy to apply though be aware of dust when applying it. I've seen the Youtube vids and claims made about the Spigen screen protectors and time will tell how good they are. For the time being it has fitted well, is very smooth to the touch and phone screen is just as tactile as it was without. It was, at £24 from the rainforest site, very expensive. That said you could easy spend half that amount on plastic ones which could be all in the bin by the week was out

Never got on very well with the plastic ones anyway.