I got my first GPS in the Honda almost as an afterthought, and like the internet and word processing, I wonder now how the heck I managed without it (answer: heavy use of Thomas map books).
Priceless features include the ability to show gas stations on the map (by brand!) in your vicinity. This has saved our bacon in strange areas more than once. Restaurants too, but unfortunately those are all generic.
I prefer my "flat map" presentation as opposed to the cool "3-D" ones common now.
It has a "bread crumb" function, which leaves little white dots on the map if you go off the road that, theoretically, helps you find your way back. I don't go off road much, so I switched it on for fun. Anyplace I go that doesn't officially match a road in the memory lays a "crumb," including parking lots. My driveway looks like a flock of seagulls attacked it.
I found that the computer's idea of the "best route" sometimes sucks, and I always preview it myself in case there's some better solutions.