I have TomTom USA on my iPhone and use it with the $20 a year traffic, works great, got my ass to Phoenix and Home, Homer Simpson was my GPS voice. The cool thing about TomTom USA is the Map is in the App, (1.4Gb file) if you don't have Data or service, you still have Navigation. I also have the windshield dock which has a better GPS chip built into it that is used when the phone is in the dock. It was well worth the $$ invested. Now my Voice on it is Mr. Burns. Makes me laugh all the time.
BTW the normal/cheap iPhone car chargers do not charge your phone while using GPS it still drains and eventually will go dead, GPS takes a ton of Battery to run, the TomTom dock does charge the phone and keep it at 100% while using the App. I ran mine for 18hrs one day. Just a heads up to check your charger, they all say they are charging but some after 2 hrs of driving will notice you have less battery than you started out with even while plugged in.
Also what is cool with this app is, my iPhone connects to my stock radio via Bluetooth, music is selectable via the steering wheel, so is calling, and when you are listening to music and running the TomTom app at the same time, it softens the music a little when the directions are being spoken over your radio then the music returns to normal volume when done. Works really well.