I hosted a presentation on Link hidden: Login to view
over the weekend, and was as always suprised that so few people seem to be aware of its existence and purpose. Simply put, GR is an attempt to standardise the way that search engine snippets and product descriptions are indexed online. It is already recommended by Google for online sellers who use Rich Snippets to present their products to search engine users, has been used by Yahoo since 2008 and Bing has integration under development. In turn, many shopping search sites that use Yahoo or Google feeds to present products by default are also using GR-friendly or at least -compatible presentation in their product results (since by definition, if they're using Google's feed they are following the guidelines Google puts out on how to structure presentation.)
GR is very easy to learn, and if you're building your own sites or doing a lot of PPC text advertising, it is very much work looking over the specification to make sure that you're displaying the correct and most relevant info to search engines and other data crawlers.