I love CoffeeScript

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 (even its creator says so).

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.)