This is one of those plugins that you don't know how much you've been missing out until you start using it. As a front-end developer who spends a lot of time designing and developing nothing but responsive sites, I've used many tools to test responsive effectiveness. Bookmarklets, actual devices, platform / browser testing services, etc.
Sure they all work, but in terms of speed and usability this is what you need for testing out your site. All the breakpoints you need to test for, all in one location simultaneously. AMAZING. toggle the break points you need and don't need. I'll be using this on all my WP projects moving forward. Highly recommend.