what is the downside of this device

It costs more than batteries. You have to rely on the sun. It takes hours to recharge it. You have to make sure you have the adapter and if you don't, they are pricey to buy. However, all things considered - it's convenient if you have the adapters and makes life a lot easier in the backcountry if you need to charge a small device like a cell phone or a GPS unit.

it's much more expensive than bringing extra rechargeable AA or AAA batteries with you when camping. It's also about the same weight as 6 AA batteries.It's also a little heavy.