The GMC10 is a very nice game accessory for a phone from the Sony Xperia series. The controller clamp fits snugly around the Dualshock 4 and the suction cup grips the phone tightly. So far I haven't had a problem with the phone coming loose or sliding; not even in 2 hour sessions. In addition, it is very handy that the mount is attached to the clamp with a hinge, so you can rotate the GMC10 about 100 degrees and adjust it as desired.
Despite the fact that the GMC10 weighs little (67 grams), you can clearly notice that the total weight, so after adding the phone and controller, is fairly heavy (445 grams in combination with Xperia Z2). You notice in the long run that the weight pulls on the wrists. How much this bothers you, of course, depends on how long the play sessions are, but the tugging on the wrists is noticeable quite quickly.
In summary, the GMC10 is a must if you really want to play seriously and normally on your smartphone. Touchscreen controls definitely don't work for me so the GMC10 is a good solution. Because you play the games with the Dualshock 4, the experience is not far from what it is on normal consoles. Now I enjoy playing games like Star Wars: Knights of the Old Republic and GTA: San Andreas; something I tried before but didn't last more than 5 minutes because touchscreen controls give practically no feedback and your fingers are constantly on the screen.
Please note, the Dualshock 4 can mainly be linked to Sony Xperia phones. Via detours it is possible to connect the Dualshock 4 to phones from other brands, but possibly with variable success. In addition, the suction cup is logically intended for telephones with a glass back. So keep this in mind.