I've tried three different cases with my week-old HTC One: the
Cimo Grip Back Case Flexible TPU Cover; the
Poetic Atmosphere Case for HTC One M7 Clear/White; and the
i-Blason SoftGel Flexible TPU Case for HTC One.
I'm very careful with my phone and when it's carried in a pocket I ensure nothing else goes in with it. My goals for a case were that it be clear to let the phone's intrinsic good looks show; that it afford at least minimal screen protection if inadvertently placed face-down; that it add little or no bulk; that it be easy to take on and off as I use a different holder in the car; and that it be priced reasonably.
First up was the Cimo. This case was rather thick but afforded the best grip along the sides. My primary issue was that the covering on the volume switches simply didn't allow one to operate the switches except with great difficulty. As I use those switches frequently, this knocked the Cimo out of contention for me.
Next was the much-ballyhooed Poetic Atmosphere. A couple of things nixed this one for me. One thing was the design element on the back of the case. Some may like it, but I don't feel the HTC One case needs any embellishment. Second, the case was extremely difficult to put on or off, making it impractical for me since, as I said, I use a car holder that requires the phone be naked, and I wasn't ready for the on-off hassle. Finally, I didn't care for the stiff back on the case, though it may offer good protection. Also, this case has a reputation for poor quality, and while the company that produces it has a sterling reputation for making good on warranty claims, I feel that their efforts might better be expended on making cases that don't require warranty replacement.
Last up is the i-Blason case. This one ticked every box. It's thin and clear; it offers excellent protection on the corners; it feels "right" (extremely subjective); it goes on and off effortlessly; and it's inexpensive enough that after I received the first one I went ahead and ordered two spares. While everyone has their own goals for a case, I strongly recommend the i-Blason to those whose goals are similar to mine.
Hope this helps.