Well, it stayed on; didn't work lose and was bone dry inside even after heavy rain. What I hadn't accounted for was the screen randomly spinning; upside-down, side-ways and back again. The Tom Tom app has no screen lock and because my bike has such a small cockpit; there's only one place it can go and it sits at a shallow angle where vibration and bike movement confuse the hell out of the accelerometer. It's useable, but annoying.
Perhaps more damping, somehow. Or a steeper angle (making it harder to see, though).