I have traveled a good bit recently.  I have used a Garmin GPS in the RV, a Toyota GPS in the wife’s minivan and also an iPhone to supplement both at times.  The biggest thing is when you have no service (read – AT&T sucks!) you will not have map data.  If you are lost, do you want to look for cellular service so you can find your way out?  NO!  A real GPS has maps stored on the device and requires no outside information.  I use the iPhone at times to look things up but it will never be good enough to be any type of automobile navigation.  Save your pennies and get a real GPS.  I have also owned a TomTom but I would recommend any Garmin over the TomTom.  The Toyota GPS was more helpful in Canada in my experience (over Garmin).

TomTom released it’s navigation application for the iPhone and the link will take you to a review of the application if you want to see it.