I recently bought the PowerMonkey eXplorer. So far I've only tested the solar charger, not actually used it in a real world situation. However in theory it should work reasonably well because it can charge the PowerMonkey while you are walking and doesn't need to be connected directly to the phone. The PowerMonkey holds about double the charge capacity of the average smartphone. So if you leave home with everything charged up the solar panel should keep your PowerMonkey topped up. You also have the option of using a second solar panel and having the two work simultaneously via a split wire to double the solar power.
The solar panel comes with a velcro strap so you can position it on your pack (or hat) so it can be working while you are walking. You can also have the PowerMonkey plugged into your phone and the solar charger at the same time if you wish.
The PowerMonkey provides enough power to properly charge or power a high end smartphone like the Galaxy S, HTC Desire or iPhone 4. Beware of some chargers they don't provide enough power for smartphones; even the PowerChimp by the same company can't be used with high end smartphones.
The product seems to be well made and reasonably rugged, I'm quite impressed (and as a plus it's a UK designed gadget).
https://powertraveller.com/iwantsome/primatepower/
Full specs on the Website