What is it?
In simple terms Responsive Web Design uses certain techniques to gracefully 'change' the design of your online content so that it is optimised for different devices and at different screen resolutions. It first came to light from an article written by Ethan Marcotte for A List Apart and has since been quickly adopted by web designers as a smart solution in changing and 'disruptive' world.
Why is Responsive Web Design important?
Because I don't know how you're consuming this content, whether you're reading it on a smartphone, tablet (landscape, portrait?) or desktop with a large monitor. Who knows how many different devices there will be in the future? Content needs to be 'device agnostic' and allow authors to publish to all those devices and screen sizes without having to repurpose and reimagine every time. So if I want this to blog post to be easy to read and to provide you with an all-round good experience on whatever device you choose then Responsive Web Design can help. In a nutshell if you want to address the diverse needs of an audience made up of many different personas then Responsive is a good way to go.
So who's using it?
Real clients and lots of them. So not just web designer show-offs, take a look.
Is it tricky to implement?
For web design professionals not necessarily, (CSS3 media queries since you ask) and that's part of the appeal, but certain design considerations need to be thought about carefully. It sharply focuses on the full set of skills used in web design including: UX (what kind of experience is this?), usability (can the user accomplish a goal?) and Content Strategy (what is my content doing, how well is it doing it?).
Can I do it to my web site 'retrospectively'?
That also depends on the complexity of your design. There are plenty of web design folks who suggest designing for online must begin with the 'mobile first' approach and it's always a good idea to consider your content strategically before getting into any design.
Does it solve all the problems?
No. Images and grids can be fluid, but (for example) Search/display Advertisements are not,so they would have to be considered in a different way. Responsive goes some of the way to addressing the multi-device issue but it's not a silver bullet.
So you should be considering it in your plans? Yes I believe so. I don't think there's any way you can ignore the whole mobile issue whatever business you're in.
If you'd like to know more about implementing a Responsive Web Design then please get in touch.