I have to disagree with the previous poster. I am a flashaholic. I own many, many lights, and yet every time someone poses the question of what light would you want with you when the you know what hits the fan, I respond with my shake light.
My particular light (the Nightstar 3
http://nightstarflashlight.com/comersus7f/store/comersus_index.asp) is not particularly bright. Not when compared with the lights we're used to discussing here on the forums that is. It is usefully bright however and it is absolutely bullet proof. Because there is never a need to put a battery in, there are no seams. The light is completely sealed. Even the switch mechanism is magnetic and doesn't require a connection pathway from the inside of the light to the outside. The website advertises the light as being waterproof to 2200 feet which seems absurd but I believe it. I've personally had mine on a SCUBA dive down to 100 feet and it worked like a charm. In fact, one of the most interesting things I noticed when I had mine in the water is that it floats. How many of your other lights can claim that? It also just feels extremely robust. It's hard to explain but in a way, it feels like it could stand more abuse than any of my aluminum lights.
As far as shaking the light, it works as advertised. Shake it for less than a minute and you have several minutes of useful light.
In general, I would highly recommend this light. For 20ish dollars, you are going to get a light that will literally last for the rest of your life. It will always work when you need it, it is indestructible, and you will never need to worry about having batteries on hand.