I've used LESS quite extensively for a big development project. It works really well, especially combined with Grunt. We have a set of site-wide .less files - chrome, forms, components, etc - then a .less file per template. We use Grunt to convert these to CSS, combine and minify to a single style.min.css. I've heard good things about SASS, and it seems that many people think it's superior - but we're quite far in with LESS, so haven't seriously compared the two.