I was convinced about CSS in JS after using http://www.material-ui.com/, which remains the easiest-to-integrate JS component library I've ever used.
Previous to CSS in JS, component libraries had to incude additional installation instructions ranging from "include this pre-minified CSS file and don't use the same classnames" to "setup a custom sass build step"; and customizable theming was a crapshoot. material-ui is practically `npm install` and go (well, it does need Roboto font, but thats a 1liner). Both theming and per-component overrides are simple and idiomatic.
Previous to CSS in JS, component libraries had to incude additional installation instructions ranging from "include this pre-minified CSS file and don't use the same classnames" to "setup a custom sass build step"; and customizable theming was a crapshoot. material-ui is practically `npm install` and go (well, it does need Roboto font, but thats a 1liner). Both theming and per-component overrides are simple and idiomatic.