Rich Snippets are used to mark up things like Reviews, People, Products, Businesses and organizations that Google will use in its search results pages to give a user a convenient summary of their results at a glance before visiting a web page.
Wow! for the kind people who have been actively involved in developing and advocating Microformats and RDFa this is really good news and its been a long time coming.
We have arrived at a new Era of SEO, as Michael Hausenblas (one of the key developers of RDFa) said "Let's call it semantic SEO", I like that, others may call it SEO3.0 or SEO++ whatever its all the same thing, Just SEO to me with more professionalism and intelligence.
For Search Engine Optimization in particular I agree with Michael, If you use Rich Snippets then you can expect a reduction in Bounce Rates, higher conversion rates, More customers, More Money. There are I think one or two down sides to consider, the popular practice of Back-Link building and Modifying Header tags may over time become less relevant because these very small nuggets of data will have less meaning compared to the data contained in a Rich Snippet.
A final thought from Jay Myers (one of the developers of the hProduct Microformat)
...microformats, RDFa, and related mark-up methods go beyond SEO and Google; they can serve as the front-end to a dynamic, smarter web of linked data. Don't implement microformats as a "silver bullet" for SEO. Implement them because your users deserve better, more concrete data regardless of the path they take to discover your products.