Responsive design has, I think, moved beyond some sort of niche novelty buzzword. I think it’s had a great effect on Web design in general and given all of us even more to think about regarding the future and what that means for our profession.
Responsive design means more work though, there’s no way around that. There’s more planning involved, more edge cases, more development time, and more testing. We’ve all go a major goal of saving time without sacrificing quality, so keeping an eye out for tools is a favorite pastime.
Testing responsive design in native environments is cumbersome and very time intensive. Demonstrating Responsive Design is a neat take on making that process more streamlined. You can enter in a publicly accessible URL and with a click of an icon have the width adjusted to the device at hand.
While not a silver bullet for testing your designs, if your dev server is publicly accessible this tool might be of great use. Alternatively, it reminds me of the Resize window feature found in the Web Developer Toolbar that could mimic this functionality for local sites as well.