The problem I've found with Microformats is that it's a misappropriate use of the class attribute that ends up causing problems on websites that have extensive templates and stylesheets. I'd personally rather have an attribute that doesn't already have another purpose as it's not likely to be abused, intentional or not. I had been looking into using RDFa however the syntax seemed burdensome and unwieldy. Microdata looks like a nice middle ground between the two previously supported rich snippet types.
The issue I found with microformats when I was evaluating it was is that there was no way to automatically transform a microformatted file into a native data structure without first knowing the schema. Writing a microformat-to-JSON parser is hard because you have no way of knowing which classes are significant and which are just there for styling.