Mine (a TomTom) has a suction pad on the bracket. I drive a Picasso, and put the bracket on the side window, which means the screen is just to my right above the steering wheel. I always remove the bracket and the SatNav when I'm leaving the car, no matter for how short a time, and we make sure that we clean the inside of that window frequently enough to remove any marks from the suction pad.
Although my SatNav is now well out of date in terms of design and style I'd prefer not to encourage a casual thief to break into the car.

Anne