Babel is awesome! 100% of the code for Cloudcraft.co is transpiled from ES2015 (even some ES7 extensions, like decorators and class properties!), using Gulp+Browserify for the frontend and on-the-fly translation in the Node.js backend. Babel allows us to use all the features of future JS, today, giving us a efficient and clean codebase. Overall, it has been an exceptionally smooth adoption, everything Just Works(tm), including debugging with source maps, etc.