Now to those that didn't understand the last paragraph, here's what it means...
Version 6 is due to be formalised any day now (well, June 2015) which means that a whole new slew of functionality is due to be added to the language (class constructors, formal inheritance, fat arrows, etc...). Browser makers will start implementing these new features once the spec is complete.
All of this sounds great, but for 2 things
1) I want to start using ES6 today
2) Even if the newly released browsers did come out today, people are not all going to upgrade their browsers as soon as the new ones come out. Typically there is about a year lag before a new feature (or set of features) has enough critical mass to be used with any confidence.
So what can we do?
Well thankfully there are transpilers which can take most of the features of ES6, and rewrite them as ES5. The 2 main ones are Babel (https://babeljs.io/) and Traceur (https://github.com/google/traceur-compiler). For various reasons, I chose to use Babel (because I found their resulting code more readable and because their gem, Sprockets-es6, seemed to have the easiest to add to our project flow). (Basically Traceur didn't work with Phantom which is what we use for testing and is actually running ES4!).
Unfortunately, sprockets-es6 had a dependency on sprockets 3 which was in beta up until this weekend.
And now it's out of beta!
As I write this I am just testing it out on our develop server and hopefully I will go live with it soon.
If you want an example of what can be done with ES6, you can mess around with it here.