I bought this for use in our motorhome. The dashboard is sloping so can't put the normal holder on there, and also can't stick the satnav on the windscreen as it would be too far away (touch screen). I stuck it firmly in one of the air vents and it stayed there happily through bumps and miles.
I was a bit surprised when I first saw that I would have to put it together, but this is easier than at first looks. What an ingenious little device!
It probably depends on where the air vents are in your vehicle, but it works great for us.
P.S. One year on, and the inside of the air vents seems to have collapsed! However, the heating/air con still work (phew!). So to position the sat nav, this little gadget is still useful as the prongs can be gripped under the map clip on the dashboard of the motorhome. It is a pity we didn't do that to begin with actually! It is a little far away, and it is a touch screen device so I suppose that was why I originally wanted it nearer.