Great answer, Ben.
A note on app rebuilding times: The ecmascript package uses the new ability for build plugins to do file-level caching in Meteor 1.2 (preview release of that work). If you’re using Meteor 1.1 (or devel before a couple weeks ago), we hope you’ll find with the new version (more RCs coming soon) that rebuilding your app is now much, much faster.
With file-level caching, Meteor provides as fast an ES6 compilation experience as you can really get. And it’s pretty fast.