[domgblackwell]

Possibly - I won't know until I've played with the code a bit. But it seems to me that Susy has brought a simple but extremely powerful idea to implementing front-end code - instead of using a presentational element and layout-related class names like "row" etc. you can use a semantically-appropriate HTML element and @include your markup via a mixin. The actual CSS implementation can be the same - it's just that it is possible to separate the semantic HTML from the presentational structure (nested selectors with included of mixins as allowed by LESS/SASS) and the CSS implementation in the generated CSS.

[perfunctory]

I'm afraid I don't quite understand what semantic HTML means. Different people seem to have different ideas about the definition of semantic HTML. Could you point me to some internet resource that explains this stuff?