Navigation provided by smartphones can be unreliable, because when the cellphone signal disappears, so do the directions.
My freebie Verizon flip phone has navigation. The directions also stop updating when I am outside of Verizon's network, even if it has a perfectly good signal and can make calls without any problem.
That aside, it does work extremely well in larger areas— it has led me all throughout Pittsburgh without a hitch. Anyone who's driven there knows that is no small feat.