Although I think this tool is fantastic, and an asset to the design community, i think it is an attempt at making the wrong tool for the job slightly less wrong. Photoshop was never designed, optimized, or planned around UI designers - it's audience is photographers.

I applaud projects like this, but sometimes its just better to use a tool that was build from the ground up around UI design, not an app that can be hacked to the point where it can somewhat deliver the results that UI designers need.

I personally use Sketch and Sketch Mirror and have found that the native support for device mirroring, retina resolutions, and CSS layer styles have become an essential part of my workflow. Photoshop gets REALLY messy as soon as you get a few iterations into a big project, and although there are hacks to get around some of the frustrations, at the end of the day they are just that: hacks.