I have the WakaWaka Power+, which I backed on Kickstarter (https://us.waka-waka.com/store/catalogue/wakawaka-power_23/) (no other affiliation). It's significantly more expensive and has less power onboard (2200 mAh vs 5000 mAh), but does have a built-in light.

Here's what I can say about it from my personal experience:

• It charges on an overcast day - can go from zero to full in 12 hours of cruddy sun exposure, much less when the sun is shining bright
• It has extremely low self-discharge, I can leave it alone for months and it will be at 90% of charge
• Very tough for how light and small it is

If the one you linked to works as well for $30, it's a steal. I'd be concerned, however, that for that price the solar panel might not be as good.