There is a new poll up in the sidebar of the site which asks:

What is your preferred CSS preprocessor syntax?

The popular ones are in there, an "other" option in case you use something else (please share in comments), as well as answers for "never tried," "don't like any of 'em," and "no preference."

I realize that not everyone has tried all of them so the end results may be skewed toward general popularity rather than a honest syntax comparison, but that's OK it will still be interesting.