A good power supply can mean life an death to your PC. If you get a cheap, poorly made unit, like Diablotek, the chances are really high that it will break, and this could really mess up the components of your computer. When shopping, never cheap out, and stick to the best manufacturers: XFX, SeaSonic, and Corsair (there are more, but these are usually the best 3).
Also add: Antec, Silverstone, OCZ and XFX.
Cheap units can work for years, providing they're lightly loaded but they ARE less reliable that quality parts. They also lack protection or it is poorly implemented so they can damage other parts when they fail, as Jacob McIntosh says.
Quite often they're also less efficient, so if you have a powerful system and pay high prices for your power a more expensive, high efficiency power supply can work out cheaper over time-besides, you want to your bit to reduce global warming, don't you?