We have them in both our cars. Since we're fairly new to the area, they have been invaluable. They're dead-on accurate, have voice-recognition, instant rerouting, etc. Ours is built-in so I can't say much about the portable units, but I understand that they are quite capable. To avoid accidents, pull over if you need to put in an address. It's old technology for me as we had multiple units in our plane (and Loran before that) long before it was available for cars.
At the prices the portable units are selling for now, you can't go wrong.
chas
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"Faith means not wanting to know what is true." [Nietzsche]