First - I did participate in, and win, a chance to own one of these mounts in exchange for my trial and review of the product. I am writing this based on my personal review and not any bias for having won the product. I am a long time member of this forum and would not want to tarnish my name for one product.
On to the review:
After receiving the mount I was a little nervous that it would not hold the phone to my controller. As you can see in the pictures it looks just like a flimsy piece of plastic. Apparently, it is 3D printed to fit whatever phone you have and the controller you choose to use it with. Well, I was pleasantly surprised to find that it held the phone just fine. It was very strong and held the phone to the controller at all angles. I was able to play games while either sitting on my couch, laying in bed, or when I took my most recent trip and played on the plane. It held on with no trouble at all. It was very easy to mount onto each device as well. A small tab at the rear attaches the mount to the controller and the S5 just sits in the top.
I am very impressed with this mount and would recommend it to anyone looking for a way to use their phone with an extra controller sitting around the house.
These pictures are from the website as my photos did not come out well. I will upload some soon