Dedicated personal navigation devices still have their place, in spite of the rise of GPS-enabled cell phones. It becomes very risky that if you are following the GPS on your mobile and suddenly an important ( cannot avoid ) call comes in, two things are highly probable , either you miss your important call or lose the way as while attending the call you cannot see the route GPS is showing.
A dedicated GPS is always beneficial in the long run . Taking the cell phone out and refitting in every time you drive your car is another issue. A dedicated GPS will stay fitted once done so.