I have a green LED Nightstar II that I bought early last year. When you shake it (with a characteristic "spanking the monkey" motion) for 60 seconds, you get about 30 seconds of useable light out of it. By "usable", I define that as being bright enough to change a fuse in a dark basement. Mine *may* be defective though; I really don't know why the useable light period is so short.
The strong magnets inside do make it hazardous to magnetic materials, such as bank cards, floppy diskettes, and computer & TV screens.
It's waterproof and floats though, so you don't need to be concerned about using it in bad weather, or dropping it in toliet bowls, tubs, sinks, fishtanks, rivers, ponds, lakes, oceansides, docks, etc.