I used CoffeeScript full-time for a year, both client-side and server-side (node.js). I completely agree with everything here -- it's a
great summary, and it describes exactly while I never, ever want to use it again.
In CoffeeScript, ambiguous code is the rule, not the exception. On top of that, things like variable capture and everything-returns-a-value introduce new kinds of bugs. The author says "realistically, the issues I have raised with CoffeeScript don’t come up every day," but in my experience they pretty much did.
But the real kicker on top of it all is that none of the stuff is documented. When you're trying to guess if you should use parentheses or a comma or an indent or a newline or a backslash or what, there are no reference documents to help you. The CoffeeScript home page just has a bunch of tutorial examples which give you an overall idea, but there's nothing rigorous, and the devil is in the details when you start combining aspects of the syntax.
In the end, I'm much more productive programming in JavaScript because I know exactly how the language works, no surprises. JavaScript has its fair share of warts, but you take a day or two to learn them, and then you never need to worry about them again. CoffeeScript, on the other hand, is just endless surprises. And I still felt that way after a whole year of using it at work. It's basically unlearnable -- no matter how much experience you have, you constantly find yourself pasting different versions of a line into the "try" tab of http://coffeescript.org/ until you get one which, through trial and error, turns into the JavaScript code you want.