It seems to almost eliminate the need for a grid system a la bootstrap, which is great because I know a lot of people that use bootstrap specifically for the grid system and throw out/re implement all of the other parts.
I would disagree here, there is another great resource online for 2 years and showing up as #1 on Google when searching:
https://www.polymer-project.org/docs/polymer/layout-attrs.ht...
https://github.com/Polymer/polymer/commit/d45aa98c9267153ce0...
I guess looking at how verbose the implementation was and what it actually gives you over learning the CSS, you're probably better off learning the CSS.
Having classes/mixins is convenient, rather than need to implement the properties "by hand" all the time. But it does get rid of most of the complexity and compatibility issues, and provides additional features (source-order independence, column-major display, ...)
That site has a few other great examples like media objects that would usually take a ton of CSS hacking but are solved easily with flexbox [1].
[0] http://philipwalton.github.io/solved-by-flexbox/demos/holy-g... [1] http://philipwalton.github.io/solved-by-flexbox/