Solar panels work best when in direct sunlight and facing directly to the sun. Its hard to say how they work on overcast since there are MANY differences in type of overcast. But yes, I have used them on cloudy days, and the difference is about null. I use them to charge a set of Eneloops when camping on a regular basis, and rig the panel on the side of my supertarp to help angle the panel to the rising or setting sun, or just lay it on the ground for noonish charging times if needed. The AA charger is nicly sized so that you can lash is on the back when hiking if walking away from the setting sun or visa versa. Also a word of the wise. Use a spare stake and some 550 cord on the windward side to prevent the panel from blowing away or fliiping over on itself if laying on the ground.
Also be advised the rollable are not recommended to be rolled tighter than it is shipped IE dont expect to wrap it around a tent pole, and you will need an external battery charger, thus increasing the weight and cost since rollables are more expensive than the foldables.
The AA foldable is a great cost (seen them at $69), lightweight, folds way small, but I do store it in a ziplock baggie just to be on the safe side. Really cant beat it for GPS, flashlight, headlamp, Camera, etc. Hence why AA is a requirement when I buy something.