The best computer is on your shoulders
Satnavs are a navigation aid and are not perfect. that said drivers have had brains longer than they have had satnavs and making sensible use of a satnav can be helpful.
Ultimately the satnav is dependent on two things, its user interface and its maps. The maps are derived from existing paper maps and so the satnav can only be as accurate as the map it has. OS maps often show minor roads in exactly the same way whether they are surfaced or not, so the satnav doesn't know it the road is surfaced either. Setting for fastest journey ussually makes it choose motorways, first, A roads second B & unclassified roads only when all else fails. Few maps have HGV information included.
the directions have to be got from the Satnav to the driver to be any use and thats the user interface. A voice is best with the screen being used only as a backup or for emphasis. The rules for looking at the satnav are the same as for passengers or the stereo or any other distraction - use common sense and only look at it when absolutely sure it is safe to do so.
Finally practice with it. check out the functions and how to mount & operate it before going near a vehicle with it. Then test it on the road. On testing a new Satnav I always do it on roads I am familiar with. If the device has any idiosyncrasies I do not want to find out about them when I am lost as well. Using the satnav on familiar routes where it can be 'over-ridden' means you can get used to the device and its controls and when you are faced with a unfamiliar road or diversion you at least know how to use the satnav to get out of trouble.