I use my cheap ebay one every time I get in the car as I use Waze (navigate app) to check for traffic, accidents, speed camera etc. It holds it pretty strong and has never fallen off.
One thing to note, as it is held on using magnets the thicker the case the weaker the attraction will be, pretty obvs probably. I use a slim case (see link below) which I prefer anyway and that works perfectly.
http://www.ebay.co.uk/itm/Ultra-Thi...s-RC20-/112346890868?var=&hash=item1a28658274