The hard part about finding a good shake light is finding one that the coil and magnet actually work! A bunch of ripoff artists have been pouring them onto the market that have a hidden battery that powers the light and the shaker does nothing. When the battery dies, the light is trash.
From everything I've read about them, the majority of them are over-hyped junk.
You're better off buying a couple real flashlights from quality manufacturers,and keeping a good supply of name-brand batteries.
Get several that use the most common sized batteries, like AA a D.
I have yet to find a shaker light that is worth a crap. Bought one of the very first ones that came out on TV years ago, I keep it in my boat, works like $hit. 1 minutes of shaking for 4 minutes of candle light.
Even the cheapest crank-lights are better IMO. I'm sure they'll break eventually but I haven't had one break yet. I keep a couple around for emergencies. One even has a USB port on it so I can charge my phone.