The important thing about SASS/SCSS (I don’t do much with less) is less about the variables and more about the mixins and now inheritance. These let you use semantic class names on the page while building a stable, debugged, reusable library of style rules. Building this library is what really gives you a leg up in speed/maintenance and because it’s pulled in as necessary, you can make the rules as extensive as you like. It really boils down to how clever you are at designing abstractions. I’m still working on it, but mine handles all my layout and typography norms as well as simple color handling (color schemes tend to vary more between projects) and I’m working on integration with yui3 so my preferred styles flow down into the CSS used in the widgets.
I was doing CSS using the techniques Michael recommends in the first comment for years but the boost in switching to SASS was huge it’s a lot like writing javascript with/without a framework back in 2005. There’s a definite benefit but things aren’t solid yet. I am pleased that I’ve done a few designs now where I’m happy with how it looks in Firefox, fire up the VM and take a look at it in IE and IT JUST WORKS.