I went really basic, and got a Magellan 315A with a complete worldwide Jeppesen Database. It runs off the lighter, and only has a small screen, but it does give excellent secondary nav capability, and furthermore, is no bigger than a cell-phone. I believe that we all operate in a potentially "hostile" environment up there, and any extra safety I can factor in is taken. I have friends who happily combine the "Good old days of flying" in their Tiger Moths and Pientenpols who also back up with a handheld GPS unit.
Cheers