I also use the TomTom App on my iPhone and am happy with it. Since it has a full basemap on board, you never miss a turn because the data network is slow. It takes about 1.4GB of space on the phone, which is fairly large, but I much prefer having a "real" GPS mapping solution in hand, especially when I spend so much time in places with really poor mobile data coverage (that would be New York City and Los Angeles).

Drives me batty that the GPS receiver turns off in Flight mode. WHY? I have a basemap installed, would love to see exactly where I am over the USA.

Flight or Airplane mode will turn off the following;

Cellular (voice and data)
Wi-Fi
Bluetooth
GPS
Location services

If the iPhone is not using Cellular, Wi-Fi, BT etc then GPS isn't required for Apple to keep track of your location.
to cross correlate against Wi-Fi locations and associated MAC addresses etc

http://www.youtube.com/watch?v=GynEFV4hsA0