The use of classes in JS, I can't imagine how useful this would be for prototyping or even production code! There are so many other features including modules (and they seem well designed).
It's also pretty easy setup-wise - just install node first then npm coffee script package manager if you're not using yarn already. Try it out before deciding whether its worth your time/money investment ;-) Its really nice when writing tests too because coffeescript lets me write more concise test functions which makes my life easier as tester :P Also great at having good documentation with example snippets included :) Very simple syntax but very powerful toolkit; there aren't any downsides yet though maybe some people might find them once their projects get bigger than 10k LoC?