I'll take that one step further and say that Compass makes CSS intelligible, reusable and efficient to write. The Compass people have taken the time to understand what the right cross-browser techniques/best practices exist for lots of common CSS problems, so I don't have to worry about it. I wouldn't do CSS without it ever again.

To Eric's point, summarized here, layout is not style, and CSS is all about style. No word yet on a cascading layout sheets (CLS?) which would provide that sort of capability.

less.js has weird issues with caching. If you use @import, sometimes it won't ever refresh the imported files. It uses the HTML5 offline caching APIs, so even clearing your browser cache doesn't help (at least in Chrome).