The microformat is a web-based approach to semantic mark-up which seeks to re-use existing HTML tags and other attributes to allows software and of course the crawlers to process information intended for end-users.
The Microformat is not an official standard, but some web sites have implemented it quite successfully benefiting from the SEs to display extra visual information appearing in the SERP.
It is important remembering that everything that looks odd can attract the searcher, and that is why the CTR of those sites that implemented Microformat have seen a traffic uplift (assuming, of course, the crawlers have successfully recognized the implementation, which is some case it is not happening).
That's why schema.org has been quite recently (27 of May) signed off by the three major players (Google, Bing, Yahoo) with the intent is to commonly define a standard to use microformats to describe elements on the page.
Why consider implementing the schema.org is a good thing?
Despite having a common schema is not a guarantee to benefit from the additional visual information, it is worth implementing the semantic representation every time the site and the circumstances allow it.
Wide acceptance of Schema.org makes it very easy for a site to implement a common syntax; this would be extremely easy for CMS driven web sites, where there is a common template and a server-side technology behind the scenes.