Just to add to discussion. The non-cheapo magnet holder (one with the rubber/anti slip cover) is plenty strong to hold even a 7" tablet (for all that my TSI could deliver). The cheapo one that I've installed in my old car (the base is basically a thick washer that moves on the ball) is much worse. I had to use sand paper to roughen it up to prevent unintended movement (and it's far from perfect).
Regarding the install process - I'd rather not route the cable on my car. While not as aesthetically pleasing it's much easier to replace the cable (that tends to go bad for no reason). Also, i believe that instructions missed important 1st step - disconnect the battery or you may end up with air bag system code (that required dealer visit).