The mapping systems are far from perfect, whenever I test it going between two points I know it rarely takes what is the best route. I also have a very annoying feature (as does my wife's Lexus, in that a footpath to the rear of our house is mapped as a road so every journey begins with instructions which must be ignored.
Having said that I have used it to get to places I do not know and in truth I still do not know where these places are I just turned left and right when it said and it took me there and back, I would now never be without it.
PS I am travelling Manchester to Milton Keynes on Sunday would I use the M6 toll road? (just so I know to ignore the navigation at that point)