I tried both SASS and LESS, but never managed to integrate them in my workflow. The things I wanted to use them for is variables mainly. But I found that regrouping selectors works way better for me. One thing is that I can change the single value in the Chrome Inspector and see it changing live everywhere.
Also, mixins look nice. But in the end, I feel it's an overkill to have a separate tool for something as simple (but not easy) as CSS. I've written my CSS and HTML in Notepad++ for years, even without auto-completion. This has helped me find ways to write my CSS efficiently, by regrouping stuff, by keeping the HTML structure clean and light, by having a clear flow in my CSS, by keeping a tidy namespace as well.
So preprocessors might just be for developers looking for writing efficient CSS for them. But that's not for me. I like the static feel of a text editor and a browser.