Responsive design presents a whole new set of challenges for today's web designers.
Statistics are showing that the public is moving away from PC computers and relying on their handheld devices for most of their needs. Thus it is important that your website displays properly on all possible hardware.
The image on the left demonstrates how this website is presented on various devises.
As you can see text and graphics re-flow and re-size to accommodate the cellphone or iPad like device they are called on.
As far as I'm concerned this is a must have feature on new websites and it should even be considered for some current websites.