I don't use my Nuvi 350 too often. But it sure was nice trying to find my turn to a new golf course last Saturday as we drove into the rising sun. Could hardly see and we would not have spotted the sign until too late.
Also used it this summer while I was in California. Nice to be able to pack it in my carry-on and have it ready to go when I got in the rental car.
Next week when I drive from Michigan to North Carolina it will help me find my hotel and then the small airport I need to be at early the next morning. All I know now is the airport is 25 miles from the town I'm staying in.
That said I think a lot of people can use a portable. Don't need it around town but if you like to head out anywhere new it sure is handy. I think a lot will be given as gifts by people like me who have found them real handy.
I would agree that for most, a GPS/nav unit is not something you would use every day. But it is far from what I would call a "novelty" item.
If you've ever used one on a trip to a place you're not familar with, you would immediately believe it is an invaluable tool for future trips. Not just for directions, but POIs are also very helpful. What I like best is there is little time wasted on finding your way, which means more time to spend on activities. I'm the type that likes to fit in as much as possible.
I live in the northeast. In my area, we can have anywhere from 1 to 7 heavy snow storms a year. But more and more people are buying snow blowers. Why? They've gone down in price signficantly over the past 15 years and it's an invaluable tool when you have 1-2+' of snow in a 2 to 4+ car driveway.
Mass adoption doesn't require it to be an everyday item, although no question something like a cell phone has a larger market.
You don't use it when you know where to turn. But for traveling to a new area or even in YOUR city, where you haven't been in a while, it is INVALUABLE. Just the confidence it gives me in being able to get around and be on time and not worry about long mistakes and getting lost. Wonderful asset. All the info on nearby resturants, gas stations (need to fill up your rental car before you return it?) etc are great also.
Someday the GPS will have a full featured travel guide with mile by mile narration of the history, geography and attractons of the area as you drive by. Sort of like having your own tour guide.