I've been using the Ram Mount x-grip for a number of years now and have not had any issues at all.
A mate of mine also got an x_grip for his little bike but his phone slipped out of it onto the ground
Luckily he had a decent case on it and it survived.
We tried to figure out why I've had no issues and he has...
We found that it was all due to the case. The case he was using unlike mine is very smooth and silky and as a result very slippery, the case just plain slipped out of the X grip when he hit bumps. He has since roughed up the edges with a bit of sandpaper and the case no longer moves at all.
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