Been using Elm for a few months now. Both in small side projects, and in a production app for work.

Many benefits over using js, and basically no downsides apart from the initial learning curve. And even that could be equal to the learning you have to do every time your team decides to add/use a new foo.js library or framework to the project.

With Elm:

  - you are confident about your code 
  - others ( and your future self ) can actually read it and understand it
  - it pushes you to think before you write
  - refactoring is a breeze, not a nightmare

I hope I won't have to use js on my next gig.

edit: Also, I cannot speak highly enough about how helpful the compiler is, and the community in the slack channel. Makes the whole experience a joy.