In CSS, styles are normally applied in the order in which the browser reads them. The "!important" keyword indicates to the browser that it should override any inherited values for this particular property, and consider this one definitive.
CSS stands for Cascading Style Sheets. It doesn't stand for Convoluted Style Sheets. "!important" shouldn't be used to hack around the fact that you can't figure out how to order your styles in a sane and meaningful way.
"!important" should be renamed to "!rethink", because if you need to use it, you're probably doing it wrong.
Comments
Unless you're using it in a local stylesheet to override broken styles on someone elses site.