I have used two different Garmins and the built in GPS's in two F350s and two Expeditions and can remember only a couple of wrong turns, both on city streets. They are very good on major highways for upcoming turns, merges, splits, etc. and helping you be in the right lane for your route. On newer ones, when approaching a turn on a multiple lane freeway for your destination the screen will show an exact picture of the freeway signs for that exit. Looks just like a photo of the highway.
Makes navigation in an unfamiliar area much easier than trying to read a paper map while driving and trying to watch for signs all at the same time. I would not be without one.