I agree on the solar charger issues, though it depends where you are walking and at what time of year.
I recently cycled from Australia to Sweden with a mate on our recumbent trikes (Adelaide-Perth, flew to Cape Town, up Africa to Ethiopia, then had to avoid Middle East unrest and started again in Turkey for the rest of the trip through Europe). Mostly it was summer wherever we were.
Being on roads, we were mostly out in the sun. In Australia and most of Africa we were in deserts and out on open roads in sunny conditions. We had no trouble keeping our phones and Mp3 players charged, though our phones were ultra basic nokias. Really my panel, a Solia with one cell was good enough to reliably keep my phone charged in Africa and Australia. Stephen's Solia was a 3 panel version which handled constant mp3 player use as well. Stephen's panel died in Ethiopia (it got too much water in it) so he threw it away and we didn't bother replacing it. Mine kept going through Europe, but, quite literally it was useless there as it would take a couple of weeks to charge with all the overcast conditions, camping in forests, rain, etc, etc. I think I charged my phone or mp3 player four times over 100 days of cycling.
You don't get shaded much when you are on roads, and we could easily mount our panels on top of our gear in optimum position on our rear racks. With a pack its much harder to mount a panel in optimum position all the time, and depending on where you are you are often walking in and out of shade. Our phones and mp3 players literally were as simple and economical on power as it gets, and the panels couldn't handle it in Eastern and Northern European summer conditions. Would have been better in the more Meditteranean climates for sure.
Don't believe the claims that all the various panel manufacturers state for charging times, etc. They are picking the best spots on the planet to charge.
In the future I'll think about using panels again, but only in the right conditions. If I walk or cycle tour in Europe or most of North America, or parts of Eastern and Southern Australia (inc Tasmania), or anywhere tropical in a wet season or heavily forested or cold, I won't bother unless I'm not worried about weight and have a long static base-camp.
For what I know of the conditions on the big three trails in the US of A (exc the desert bits in the south west, I've never been to North America myself), I'd personally not bother with a panel on a thruhike, unless I got a custom pack made of embedded panel. It would really need to be that big! Wait for it though...I give it less than a decade before we start to see commercial rucksacks made with embedded solar panels.