Drove for 21 years without one, and now have one both for private journeys and one for work. They are very useful, especially if (like me) you end up having to go to lots of places you don't know. You can avoid getting stuck in one-ways & having to stop frequently to look at the map.
HOWEVER... you should never follow one blindly. Read the road and the signs, too.
Two examples: my work satnav keeps sending me down a side road to get onto one of the main roads out of my village. Trouble is, that road gives access to the main road by pushbike or on foot, but never has done so for vehicles.
Why doesn't the mapping source know this?
Also, beware country lanes. You can be looking at your screen and it implies going straight on, with no indication of right of way. What you don't know, as you hurtle along, is that round the next bend is a give-way in 50 yds.
You need common sense & to use a satnav in conjunction with road signs and your senses.
That story of using a road posted as "Unsuitable For Motor Vehicles" is plain daft.
On the other hand there is a road in the High Peak I have used once before. Never again. The satnav sent me down it. There were no warning signs. All was good, but then the road surface deteriorated. Only gradually, though. It was a gravel track next, then a rocky one, then it became uneven. By the time I left it, it had huge craters in it & had to be negotiated at crawling pace to avoid ripping the sump or exhaust off...
But to me credit, there were no signs telling me this would become the road from hell!
I don't go that way anymore...