I'm in the process of ripping the last bit of coffeescript out of our code. The marginal benefits of the language don't exceed the costs of having another build dependency, and another syntax to understand. For daily work, I already have to switch contexts between Javascript, CSS, SCSS, HTML, rHTML, HAML and Ruby (not to mention other tools like Bash, Rake, etc., along with API knowledge like the Rails API, jQuery, and all of the various un- or poorly-documented gems). Ultimately, it's a huge mental burden. Every new thing is a new thing that someone has to understand to be productive.
To justify the extra mental cost of another piece of technology, it has to do something pretty amazing. A minor syntactical transformation of javascript just doesn't cut it.