I have both the black one and the transparent one.
They are really thin and add minimum bulk to the phone. However, they also barely protect the phone at all, probably only good for keeping the back of the phone from scratches.
At the back, the case is level with the red/blue ring around the camera lens, i doubt it protects the lens at all.
The transparent one has a frosted feel and is still pretty slippery, but it's better than without a case at all.
I would recommend the black one as it gives a rubbery texture and it's pretty grippy.
Overall i love how the cases manage to give me minimum protection and bulk.
i chose the One S over the X partly due to it's thiness, so i bought these cases to keep it that way.