Steve. I think I should mention a popular miss conception concerning truck mode on sat navs. A truck mode like what was on the garmin and tom tom , ie choose car truck or pedestrian, does nothing to alter the actual route, merely the time, it does not change the route to avoid side street, they never have.
A proper truck mode, will be one that asks you to put in details of your vehicle ie weight height length, this will indeed give a route that will be based on the info you give to avoid possible problems ahead. so be careful if you think a sat nav which has a truck mode without you having to enter any data will avoid any road pitfalls, as they do not.
And another point that people seem to be missing is that a sat nav, actually is a map a moving map!and can be used as a still map. which you can use just like your out of date paper map to look at the route and plan it, you can also make changes to your route and avoid certain areas roads or streets, tolls motorways ect.
Something you cannot really do with all concious awareness unless you have an excellent memory or keep reading written down notes whilst you drive.
The newer version also have clever routes ability too avoid congestion, and some also have life service functions, like the one I have,and which diverted me off the A438 at stoke Edith near Hereford onto the A4103 because of a serious road crash further up the A438.
No they are not perfect but are of dam more useful than the old paper map, its the user who might be the weakest link not the sat nav.