Keep in mind, this is not just simple image scaling. It's truly using different source images with the
srcset image parameter. It's an awesome effort and it's not simple. Better yet, the plugin aims to be an “activate and forget it” feature, whereas most efforts I've seen require shortcodes or other non-sufficient methods.
Responsive images will eventually need to be part of the core image selection process, I believe. This is a great plugin to get involved in if you're using large images on your responsive websites (hint: that's probably most of us).
Tim wrote an outstanding breakdown of the plugin and how it works on Smashing Magazine. You can also check it out on WordPress.org or Github.
This effort required months of work, and they even got feedback from some of the core team in the WordPress Slack channel. This is the best effort yet for WordPress responsive images. We need to keep pushing and iterating, and this is an awesome start.