Of course you can not expect too much at this price.
So on the back of the original S5, the suction cup holds absolutely not, therefore not recommended. I have a TPU Silicone Sleeve Mumbi which, however, does not hold a pity. It is up to the rough and covered with Strukutr surface.
A little tip, have me for 2 ne cheap transparent cover of something harder plastic concerned which is completely smooth in Tedi.
This shell of the suction cup without any problems holding. The surface must be virtually as smooth as the back of a Galaxy S3 or Note Tab 10.1 as an example. But would have written like this from another buyer "Klaus L." the thing fasten a little pimping and a universal mount.
Therefore, the mount is to use already only on certain surfaces.