I've been using the Babel transpiler to get async/await, and the code is so nice. Showed it to a JavaScript-hater and he gushed over it for 10 minutes -- then repeated the same comments to his boss (my client), which was awesome. :)

But I'm hearing that both the Babel and TypeScript implementations leave a lot to be desired with performance. Not so bad that it gave my use case a problem, but I use NodeJS, in part, for performance.

I'm also targeting browsers (and hybrid apps) with some code, and I've recently jumped from Babel/ES6 to TypeScript, so I can get the support via generators, but again I'm hearing of performance issues.

Getting first class optimized browser support is where I'll feel it's truly awesome.