I haven't switched to coffeescript but I have worked on several coffeescript codebases. Its not the end of the world, but I don't prefer it. It is an unnecessary level of abstraction and is fragmenting js. It also seems the coffeescript users tend to use haml and sass as well. When I am working on those projects I feel like I am drowning in preprocessors and frequently encounter cryptic errors as a result.
I can't speak for haml, but sass is certainly a welcome tool in dealing with CSS, since it mostly uses CSS syntax, with sensible nesting to keep things orderly. sass takes what you already know and kicks it up one notch (perhaps two, if you want to use variables).
