I've tried a few cases now as there are times when I am out at work and need to have the phone with more protection than just a screen protector.
HTC Double Dip Flip case. My phone doesn't sit in it correctly and won't 'snap in' at the corners. This renders the case useless as the phone could fall out.
HTC Double Dip case with the red top and black bottom bars. This really doesn't play very well with my screen protector as it pushed it up at the corners. Coupled with the fact that it is hard to put on and take off and will doubtless scratch the phone with constant removal. A shame as it is quite compact.
TPU 'gel' cases. Neither of the TPU cases I have have afforded me access to the power button. Both of them make it so that you really have to force the button down to turn on or off your device. These cases are about as useful as a chocolate ashtray on a motorbike because you just can't turn the phone on or off without pushing hard.
My favourite case for my iPhone was a Speck Candyshell and I am glad to see they are bringing one out for the HTC One.
It's not available in the UK yet so I may just end up buying this from the USA instead.
EDIT: Not at almost 65 USD form the Speck website I won't! I'll wait for it to come to these shores.