i find that people that regularly use SAT-Nav, become a bit dumb when it comes to knowing your way around.
i dont think you should be using it on repeat journeys.
i also noticed that it reduces concintration levels in general, (although you're better in complex areas you dont know).
Sometimes, the road is not even in the TomTom, so you spend ages trying to find it, going through all the options, different spellings/towns etc.
i do have Sat-Nav though, (courtacy of CrackheadsRus

) ~ so many sat-navs get stolen. So i keep mine at home, unless i'm going somewhere special/far.