After seeing the value of GPS while riding a brevet at night, I am considering purchasing one. Garmin seems to have the greatest variety of handheld units, but I have Delorme mapping software and have been giving thought to the Delorme Earthmate; another possibility are the Magellan handhelds.
For Randonneuring, it seems to me that the training type of GPS, such as the Garmin Edge, would be less useful than models that can show a map. Then battery life is a consideration. Replacing batteries on a long brevet seems more time efficient than trying to find an outlet and waiting for a recharge.
What has been your experience with using a GPS as a navigation aid on brevets? Have any makes or models worked especially well for you?