>> If you don't it may be best to stick to the GPS.
If you don't know the route, it's best to check map(s) and use streetview to look for landmarks before hand. GPS firmware are still coded by human and is most certainly buggy; the incident involving UK ambulance that went off course by 200 miles comes to mind... (ref http://news.bbc.co.uk/2/hi/6200054.stm)
Alice the controller losing control. Wow. The fist of doom will redirect Dilbert's phone towards the nearest wall with great speed. What a horror show for Dilbert - he has to work with this woman - I have every sympathy for him....
If you know the route, use your knowledge and ignore the GPS
If you don't it may be best to stick to the GPS.
If you are driving something other than a car (like you hired a large motorhome) double check the route as you don't want to go down a narrow road. And keep an eye out for signs that give height and width limits.
Seen many a lorry jammed under a low bridge because they didn't bother checking the height limit.
My gps on my phone has been invaluable. The worst one though was when I missed a turn and had to turn off on a different road. The gps kept rerouting, and told me to make a u-turn. No problem, but then after I made the u-turn and headed in the other direction, it again told me to make a u-turn, into the direction I just came from. Basically if I would have followed it's directions, it would have just kept me going in a circle of continuous u-turns. Once I got back on the main highway, it got itself straightened out.