I have four cases for the HTC One for your consideration. I have purchased and used each one of these cases for at least one week each.

They are, in no particular order;
- HTC One Double Dip Flip Case HC V841 £24.99
- HTC One Double Dip Hard Shell HC C840 £17.99
- Belkin F8M570 Micra Fine Ultra Thin Case for HTC One – Clear Steel £24.99
- Casemate Barely There HTC One Case – Black £18.00

The first two can be ordered through HTC’s own website, although the order was fulfilled by Expansys. The Belkin and Casemate cases are widely available by doing a Google search.
Why Use a Case?
Firstly it is worth considering why I (or you) might want to use a case. The One is actually very well constructed and made from fairly tough materials with an aluminium back, Gorilla Glass (GG) front and a small amount of polycarbonate around the sides. I know that GG is very tough, I’ve seen the impressive demos where people attack the front of a phone using implements such as knives and keys, even a drill!
However, firstly I still don’t like placing my expensive device down on a hard surface, rightly or wrongly I imagine a grain of sand (or something) coming between my One screen and whatever it is resting on and this just makes me cringe. Secondly, although the aluminium and glass finish is lovely to look at, it is (just like my previous iPhone 5) prone to being slippery and can be easily dropped. I know, I’ve done it more than once, thankfully with no damage as it fell on a forgiving surface.
So, when wishing to protect my phone my first requirement is for something that gives me an improved grip and the second is for something that comes between my precious and whatever it is resting on. And, most importantly, it must preserve the good looks and pleasant tactile feel of the device whilst doing this.
To that end, I tend to lean toward the thin end of the case spectrum rather than the tough as nails Otterbox type of case.
Double Dip Hard Shell
Comprising of a central piece that the One slips into, then with two clip on ends. The red end going on the top half of the phone and the grey end at the bottom. It is nicely constructed and fits well, providing wrap-around protection to the back, top, bottom and sides, but not the front. This allows the One to be placed face down without fear as the raised edges keep the glass front from touching the surface it is on. It also provides a reasonable amount of drop protection, particularly on the vulnerable corners.
The look of the case is definitely fairly unique, in a sea of single colour cases it stands out, and with the different colour ends when you pick it up you immediately know which end is up and which down, not so obvious with the bare phone.
On the down side, I’m not sure I like the way it ‘disguises’ my good looking HTC One. I would actually prefer it either all black or see-through.
Double Dip Flip Case
This is an intriguing and well made piece of kit. It’s premise is simple; it provides a form fitting shell for the back, top, bottom and sides (allowing for the cutouts), with an attached wrap around screen cover. The inside of the screen cover (and half the inside of the back) are lined with a vibrant red soft-touch fabric. It also has a neat trick, as the One can be propped up in landscape mode using the screen cover as a stand. Combined with the highly effective front-facing Boom Sound stereo speakers this makes for a good way of listening to music or watching a film.
The One is surprisingly easy to slip in and out of this case, as the shell portion doesn’t fit as tightly around the edges of the One. Don’t get me wrong, it is not loose, it just doesn’t wrap around the front as much as the others. It doesn’t need to as it has the attached screen cover to protect the front instead.
One slight annoyance, at least for me, is that in use you have to do two things to get to your One. First, open the screen cover (it flips around to the back), then press the power button to switch the screen on. As someone used to my iPad Mini switching on, just by opening the smart cover, this is an extra step I don’t like.
Belkin Ultra Thin Case
On first glance this should be the case that I like best of them all. It is very thin and light, but still wraps around enough of the sides to raise the glass of the surface when placing the One face down on a surface. It provides the right amount of grip for when handling the phone, so I’m less likely to drop it when going all butter fingers.
It has a pleasant finish to it. Although available in several different ‘colours’ this particular one is called the Clear Steel and is darker grey (steel) at the bottom end fading to a semi-transparent finish at the top end. This shows of the form and finish of the HTC One, better than all the others. It fits well and has just the right cut outs for things like the camera, volume rocker, rear microphone, etc.
Where it falls down for me is in use. It is slightly too thin and flexible. This makes it easy to get on and remove but also means that it creaks slightly in the hand when holding the phone. Over time, this bothered me more and more. Shame, as otherwise it was almost perfect.
Casemate Barely There Case
Similar in concept to the Belkin case, but of slightly thicker and more solid plastic, it immediately overcomes the annoying creaking issue I had with the Belkin. It provides the same, or better, level of protection but feels much nicer in the hand. It has a cool and matte finish to it that is non-slippery and pleasant to hold.
The coverage around the volume rocker is better, but this leads to another issue as you have to be very precise in order to operate the rocker. The whole case is also very slightly thicker and therefore more bulky than the Belkin one. Also, rather oddly, the cutout on the rear for the camera and flash is rather bigger than it needs to be. Perhaps it is to aid in removing the shell?
Conclusion
Of all the above cases the one I am still using is the Casemate Barely There Case. It is the closest to my ideal, although still not perfect. Indeed, I am awaiting delivery of what may be my ideal solution. Not a case at all. Instead a clear skin for the back, to help improve grip and slightly protect the rear, and a tempered glass front cover to protect the screen. This combination will leave my HTC One looking as close to ‘normal’ as possible whilst still addressing a couple of my concerns. I’ll let you know in due course.