I have driven with one.  Several times.  And they provide no more
information than a good road map and a bit of common sense.  And they
do tend to cause the driver to lapse into following directions without
thinking about consequences.  Not all the time certainly.  But I just
don't see the benefit.  And they can lose signals in cities with tall
buildings.

Roadie been around a while and posted some good stuff. A bit opinionated
maybe but not trolling, so they can stay out of the bitbucket.
I kind of agree with them anyway. If I have to travel unknown territory,
I prefer a full map to a mapquest printout or GPS picture. Once I have
an overview of the area though I can put the map away and follow my
internal compass to get where I'm going. Sometimes have to pull over and
consult the map again but that's just getting old and advanced crs
setting in.
If I have a GPS in the car, I won't turn it off and put it in the glove
box though. Better to hear a turn coming up than squinting at signs
through the pouring rain in the dead of night.
I bought a TOM TOM 700 for my parents for xmas a couple years ago.
They're both retired, have motorhome and the time to travel. Dads ex
engineer so I figure he'll latch onto it. Moms afraid of anything
technological so I'm guessing she'll never touch it.
I remember as a kid, mom taking us somewhere across town and getting
lost, finding her way out of the housing tract we just drove into.
If it weren't for my keen sense of direction and internal compass, we'd
still be driving around looking for the main street. *g*
Short story long... was I surprised when I got an email from mom talking
about how much she loves 'Thomasina' (yes, she named the Tom Tom... and
she does do email)
One of her biggest navigational challenges is getting somewhere and not
being able to turn the directions around in her head to get home. Now
she fires up Thomasina and hits 'take me home' and follows it until she
knows where she's going.
She sings in the choir at the college and one of her assignments was
deliver tickets for an upcoming performance to the music directors at
some of the local high schools. She sat down with a map and Thomasina
and programmed in a route to hit them all. Made the deliveries without a
hitch.
No way she would have accomplished this on her own, without a meltdown
along the way...
Recently, I went with them to a banquet over in Long Beach. A 20 minute
drive in familiar territory. I suggested bringing Thomasina since I've
never traveled anywhere but from the kitchen to the living room with it
before.
Worthless thing couldn't get a signal and didn't finally figure out
where we were until we had made a couple U turns, found the place and
were pulling into the drive... And then the GPS was telling us to make a
U turn and go back the way we came.
Whatever... mom loves it so that's all that matters.

Ah, yes it is good to read such useful and well thought out
commentary.  Yes, you have provided a lot of insight into the question
of whether GPS is really worth several hundreds of dollars or not.