I haven't used coffee script myself, but I think you'd be hard pressed to find someone that wrote something in CoffeeScript and regretted it. The biggest arguments against using CoffeeScript (or any language that compiles to JavaScript) that I seem to keep hearing are:

1. JavaScript isn't a very readable language. (I think this is untrue. I've seen some incredibly well written / structured JS) 2. Not everyone knows CoffeeScript, so when you put a project up on GitHub and it's written in CoffeeScript, it's difficult for people who only know the syntax of native JS to understand what's going on. (To counter this - a lot of projects that are written in CoffeeScript are bundled with the compiled JS code as well - which also happens to be very readable IMHO).