I've used Coffeescript on a couple large Rails projects, and I'd rather stick with Javascript. I haven't encountered any terrible debugging hurdles, but it just seems like obfuscation. And unlike one of your other respondents, I love Sass and Haml. I have nothing against preprocessing per se, but I haven't yet figured out what value CS adds. I appreciate easier iteration, but that's about it. But considering how many people rave about it, I'm willing to learn....