I'm using TypeScript everyday, so I might can give a good answer to your question. They have just finished support for ES6 modules, template string, let/const etc. ES6 classes have been supported for a long time and most of the syntax sugar of ES6 I can think of is supported. On top of that, they are about to release v1.5 the big thing there is support for decorators.

What is not so good with TypeScript is that they have a rule of not polyfilling things. So if you want any new ES6 methods for array/string etc. You would need to polyfill them yourself.