I'm a "use tables for layout occasionally" kinds guy, but this isn't the same. This isn't a CSS replacement for tables, this is a standard set of styles to assist in doing a grid-based design.
The point of using CSS for layout is to have the markup be relevant to the content of the document, not for it to be relevant to the styling. Just as having class names like "blueText" or "thickBorder" is injecting style information into the HTML, so is having class names like 'g80' or 'g940' or whatever.