Did you also consider Flow[0] as an alternative? I'm now using ES6 with Babel to build some small side-projects and it has been a great experience. But as the codebase grows, I'd appreciate to add some Flow type annotations. Typescript looks great but it's still not JS. I wonder what will happen to all those compile-to-JS languages once ES6 becomes supported everywhere.
[0]: http://flowtype.org/