I agree that part of the adventure is ending up in places to which you never decided to go. I've never regretted it. But that has nothing to do with whether you should take a GPS.
I recommend the Garmin eTrex Vista HCX. Best compromise for maps, battery life, weight, and functionality. The maps that you can get for it are mostly USA and Europe. I like to say they are 99% accurate -- that means 1 in 100 roads are wrong! Along with mapsource on your portable computer, you can find great rides in unknown places.
There is a valuable function that is hardly used. That is for geocoding your photos, which means embedding into your digital photo file where the picture was taken. How many photos do you have to which you say, "Where was that taken?" If you go to this page you can see an example of what I did on my last trip. http://greggferry.com/travel/2009/07/19/
So gadget geeks, get a GPS