As a web designer working with legacy code, this speeds up my workflow considerably! It lets me collect all the classes used on a page into a style sheet (which is already formatted, so all I have to do is save). Makes it so much easier to determine if a page is inheriting styles from other sheets. When you've got CSS being imported from all over the place, it's a pain to hunt through looking for that one class that could be messing everything up.

If you work with complex .NET applications with views, like in Visual Studio, this extension is for you! I kind of wish it also spit out the properties (building a complete sheet) though.