It was the default for a while for Rails apps and Wiki tells me GitHub & Dropbox adopted it before moving to Typescript. I think for Rails shops it was popular and there was a brief time where it looked like it might become the thing to do on the front end. But I don't think it ever got the interop with existing JS right.

Ultimately, improvements to JS came along and took the wind out of CoffeeScript's sails. Then TypeScript came along and killed whatever interest remained.