I find that our large screen Tom Tom works well for me. In particular, it changes the view at junctions so if you need a visual clue as to where the next road really is, it is already on the screen in large scale. I leave the volume set quite high and mostly rely on voice commands from the box - after all a junction is where you should be looking at the road and not at a device.
I do set it so I can see it easily so that I can easily check ETA when cruising as, for me, one of the benefits of a Sat-Nav is keeping my speed down on longer journeys by removing the urgency from relying on a very rough journey time calculation. I now allow enough time and believe I have done so through the Sat-Nav's traffic information system and auto re-route function.
Our recently acquired Mini Clubman has a built in Sat-Nav which is very much inferior to the Tom Tom - fixed view unless you navigate through menus which, as others have said, is a distraction whilst driving.
