If someone in the know could give me a rough estimate of "When can I expect to be using ES6 to develop applications (at least for modern browsers)?", that'd be nice.
And I'm not talking Babel or any other tools like that, I'm talking straight up ES6.
A good while yet, I'd expect; a lot depends on Windows 10 uptake. The browser-formerly-known-as-IE is currently running ahead of Chrome and FF on ES6 implementation, but if everyone sticks with Win7 for the next half decade it won't amount to much.
But seriously: use Babel.
Or better yet, use jspm with Babel and use ES6 modules for everything.