Why not just buy one of the awesome Nav apps already available? Too cheap?....
Because google nav *****$ all over any other nav app out there! Esp if you compare free to 69.99 or 99.99.
Now granted you need a cell signal, that would be the only downsize i could see using google nav. But it does cache you whole trip as far a directions and voice.. so even without a signal you will still be told and show(Sav map terrain data) where you are going.