I was plotting a course to build an experience like this into Firefox a while back. I hope it will eventually come to pass as part of the browser some day, as I believe that’s the best way to make it feel like an integrated experience with the other tools already in the browser.
For now, it’s nice to see tools like Polypane, Sizzy, and others experiment with similar ideas as separate tools. It’s quite nice to see at least 2 views at the same time when working on a responsive site.
Seems neat. I’m not “sick of resizing dev tools” though, so I wouldn’t use this.