I actually haven't used the latest proposed versions of CSS Modules directly myself, but we use something really similar to it at Facebook. Tim Yung built Facebook's version of CSS Modules in 2012 to be used alongside XHP, and later with React.
One thing that we did differently at Facebook, is we disallowed importing CSS rules into any file other than the XHP/React component. That helped enforce that a component should expose the right API via it's props to be styled correctly and that other components couldn't import a rule in order to override it. That also meant that we never needed to add anything magical to JavaScript to "import" a CSS file, instead relied on a naming convention where Button.js would sit next to Button.css
The last benefit of modular CSS that we really liked was the ability to minify the class names themselves. When looking at the DOM in our development environment we see full class names like ".Button_ColorBlue" but in development we just see ".a5u". That helps on file size, and ensures no one can "guess" what the actual class names in the browser will be.