I've seen people try on the trail - the answer, when I ask if it works, is a general "meh, not really."

I picked up a Brunton Inspire (a battery pack you charge at home) on sale so I will be able to complete a review of a GPS app with my cruddy phone, Sir Dies A Lot, which has an average battery life of half a day or so, maybe 8 hours if i remember to hit the app killer app often enough during the day. It works - charges in an hour or two, gives up to four charges, so it might get me through the four days in Point Reyes this weekend. Probably about the same weight as the solar panel options.