I love CoffeeScript

Edit: I've rethought some of these opinions in "Thoughts on CoffeeScript after ECMAScript 6".

In short:

CoffeeScript is just a better language than JavaScript. It's on par speed-wise. It's more readable and writeable. Fewer characters have to be written for the same result. Less code and better readability means fewer bugs. CoffeeScript is a better language than JavaScript.

Debugging it is a pain if you don't know JavaScript (and even if you do). Getting initially set up is annoying. You need to know JavaScript in order to use it well.

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.)