I'm currently using Moment.js, i18next and Numeral.js with AngularJS. I wonder how FormatJS compares with this. At least one benefit FormatJS could have is having a unified collection of translation files.
the main benefit of formatjs is that it offers a declarative syntax at the template level, which simplifies things drastically. we don't have an integration for AngularJS or Ember just yet, but we are planning to do so very soon.