I've been developing only responsive websites now for over a year, and it is becoming more and more important these days. There are a lot of reasons to go responsive, and there is a lot of design and UX/UI thought that needs to go in to it.
Ever had trouble trying to explain to a client what responsive design is, and how the content flows between different device sizes. Unless your there in person, it can get tough, e.g.: over email or on the phone. This awesome little interactive simulator from Fine Citizens will help any client visually understand what a responsive site is.
Responsive is all the rage and css media queries are how its down. They allow you to target CSS rules based on – for instance – screen size, device-orientation or display-density. This is a great resource of media queries which target specific devices you might want to customize your web experience for.