I'm in the process of introducing Scala.js to the front-end of seventh sense. My navigation and several display components are in production already, with more coming all the time. Once you understand the interop rules and get a unified toolchain in place, it "just works", and as others have said, having access to scala typing and the same, typed, codebase on both front-end and back-end is awesome.

I'm currently using Angular, React and Moment.js on the scala side, so I can confirm JS library interop is possible.