When I started to read this thread, my first reaction was "never on my bike".
Then I read a few posts and changed my mind to "well, maybe on my bike".

Then I went to Reno for Street Vibrations. I studied the map the night before and got the roads all lined out in my head.
In familier territory, I was day dreaming of a resturant I wanted to go to, and completey missed my first major turn off

... so we went on anyway, adjusted in my head for the new route.
got back on the highway I was aiming for. then after a couple hours, thanks to my 'internal compass', I noticed that we had been going south east for entirely too long, when I knew Reno was north and east!!!! so I Pulled over and looked at a map. I had missed the turn about 40 miles back!!! so I re-adjusted my route again and we headed up a highway I had never been on before and got to see a lot of great Countryside.
Moral of the story????
With a GPS I would have gotten there 2 hours earlier and taken the route planned.
Without the GPS, I got to see a lot of new little towns etc and still got there before dark.
OR--> with GPS I would have seen some other cool roads that branch off the main highway and still gotten there before dark.
But
I am still considering it. Like someone said...ya dont hafta keep it on all the time.