I use to be one of the biggest defenders of CoffeeScript: http://procbits.com/2012/05/18/why-do-all-the-great-node-js-developers-hate-coffeescript
I've since changed my tune quite a bit. It's not that I think that CoffeeScript is inherently bad. My problems are as follows:
1) In the Node.js ecosystem, if you're going to write modules for others to consume, you should write them in JavaScript if you expect some sort of adoption or contribution.
2) CoffeeScript can deceive JS newbies into thinking that it solves JS's problems. You should really have a solid grasp on JS before starting CoffeeScript.
3) I since came to the conclusion that CoffeeScript doesn't offer much more than JavaScript. The lines of code that it tries to convince you that it saves, is only true if you compare CS to the compiled JS output. But in practice, the margin is much smaller.
I use to write everything in CoffeeScript and have since converted over exclusively to JavaScript. If you want to write CS for your own app or your team's app, go ahead. But if you write OSS, don't expect as much contribution as you may get as if you wrote it in JS.