Few questions, how does this product hold up to water? say on a backpack hiking in the rain.
If it is raining will it still charge without direct sunlight or will UV still produce a charge?
Is the charge immediate or must this device be in the sun for sometime to say charge up the cells?

I would call them water resistant, but not water proof unless you tape up the connection points. I've found that it will still charge on overcast days and I've even started putting it inside the tent while I'm out for the day and it will charge on sunny days.

The charge will be immediate, but it will be slower than plugging the device into a computer or wall socket to charge since there is less current being generated. Also, Photovoltaic Cells (PV Cells) like this operate by converting the Photons from light into energy. When its rainy and overcast, the light from the sun is more diffused, and thus the photons are more diffused, and the device has less photons to absorb and convert into electricity. Panels like these are good at converting Infrared Light and Natural Sunlight into energy, UV capturing panels are slightly different in design and relatively new and expensive. Meaning this is less useful in UV or low light conditions. I cannot speak to its water resistance. Hope that helps somewhat, good luck.