Responsive design is taking over the mobile industry and it should. Having a separate subdomain for an m.url for your mobile site is not user friendly for your customers. With a seperate mobile website, the mobile device will redirect users to your mobile site subdomain potentially confusing the user, possible causing them to leave the site before they have even had a chance to use it. A responsive design allows your users to view your website in a mobile friendly way without redirecting the users to a subdomain or separate mobile site.
How does Responsive design work? Essentially a responsive website adjusts to the size of the browser using media queries and other technical web design tricks to determine the size of the browser (desktop, tablet, or smartphone) and displays the content of that website in a more user friendly way for that browser size. Here is an example of our Trellis responsive website and how it looks on desktop compared to mobile.
Trellis Desktop Website Version
Trellis Mobile Website Version
As you can see the website adjusts for the size of the screen. This is just the smartphone and desktop version but it also adjusts for the tablet size as well. There are a few ways to build a mobile responsive website in an efficient manner. One is to use a theme that is already responsive on an open source platform, such as WordPress or Magento. There are many responsive frameworks and themes and if you do not want to use a theme that has a lot of features but want something responsive, Foundation and Bootstrap are great ways to build responsive websites without having a bunch of unnecessary features built into a theme. You can use foundation and bootstrap as a WordPress or Magento theme, as well as just a front-end framework to build a website from scratch or on a MVC framework like rails or laravel.
There are still a few problems with responsive design in that optimizing your site for all screen sizes for both content and images will never be perfect because there are so many variations of browser device sizes and browser types like chrome, Firefox, IE etc. However, it still is a better way of developing a website for mobile in that you are just adding more functionality to your existing site, rather than developing a completely separate mobile site to be viewed on mobile devices with a m.url subdomain. As the web grows and responsive becomes more commonplace, most of the problems with responsive such as finding ways to reduce image sizes to the correct browser size properly will become easier as more developers find better ways to do this and share their knowledge with the developer community.