I have the official (more expensive) Garmin one and it slid off the dashboard regularly in my last car. So much so that I haven't even tried to use it in my current car. I use the simple windscreen suction mount instead that comes free with the Satnav.
Only thing I'd say is don't do what I saw recently in a car park. A young lady got into her car next to mine and stuck her satnav on the windscreen directly in front of her face!!!!! Not something I'd recommend if you want to live to become an old lady my dear.