Responsive design is one of the best things that ever happened to web design. With responsive designs we can use one codebase to support everything from small inexpensive Android phones to 5K Retina iMacs. Not only is that great for mobile web users, who get usable sites across all their devices (assuming responsive design is implemented properly). It’s also great for developers and the people who pay them — no more mobile-specific sites.
Responsive design makes the web more usable, more flexible, and more accessible for users, and it makes building websites that support all devices more efficient, more cost-effective, and more enjoyable for developers.