I've tried to learn coffescript at one point, but never ended up using it for larger projects. Most people I've worked with were using plain old JS. A lot of the good parts found in coffeescript will be available in ES6. For example, there is finally going to locally scoped variables and constants. The short hand function definition with arrow will also make code cleaner. I am thinking about to switching to ES6 when there is better browser support for it.
That's handy. didn't know those existed. From this page, http://kangax.github.io/compat-table/es6/
It appears that Babel has more of ES6 implemented. Have you tried using Babel/Traceur? Did you like it?
Haven't used neither of them yet. Plan to implement Babel(seems to be the favoured compiler) alongside PostCSS(basically same thing only for CSS) in next project. Although I haven't tried it yet writing next generation JS should be a strong positive especially if Babel does a good job of compiling your code to ES5. If you are interested I would suggest doing your own research to the pros and cons. Or just give it a try.