Great Promise Rated 4 out of 5 stars
This addon overlays a ruler on top of your viewport. It makes it somewhat easy to analyze a site as it crosses multiple breakpoints. The ruler values are a little hard to read, but it does the job. I was hoping the ruler would surround the viewport instead of overlaying on top of it so I can see the entire site (including the portion underneath the ruler).
This app has great promise and I look forward to enhancements.