I won't use Sass, Haml, LessCSS or things like that until I see Sass, Haml and LessCSS in Firebug or View Source (or whatever your choice of web development tools). The claimed benefits are nice in theory, but I think it is a debugging roadblock to have to mentally translate and associate your source code with what the web server serves.
If I code in CSS and HTML, that problem is practically nonexistent (excepting things like templating). The CSS and HTML the web server serves are the CSS and HTML in the source code.