Last Christmas, I got my wife a TomTom One Europe GPS car navigation system. She had been lusting over this little device for quite a while. I did some research and all the reviews were very good for it.
She doesn't really use it a whole lot since we usually know where we're going. However last week, I borrowed it and took it to Austria for a ski trip. It's the first time I ever tried relying completely on a GPS system to get me somewhere. I must say it worked really well and both my travel companions are now considering getting one too.
It's a bit weird but I found you soon start to completely ignore road signs and just follow instructions from the unit. I can see it being a bad habit if you find you've programmed your destination wrong! I also found that having a GPS unit makes you feel much more confident about not getting lost and that makes you more willing to explore an area. It also makes for less arguements between the driver and the navigator! Highly recommended.