Edit: I've rethought some of these opinions in "Thoughts on CoffeeScript after ECMAScript 6".
Edit: There are also features of ECMAScript 6 that won't be supported by CoffeeScript, which is a drag. Another disadvantage.
But I think it's worth it. CoffeeScript is a better language.
(Note: I'm making some pretty bold statements that are about 95% true. For example, there are cases where CoffeeScript is less readable, but I think that's rare. I still love you, CoffeeScript.)