Elliotte Rusty Harold makes some excellent points regarding microformats, offers a macroformats alternative, and even finds time to advocate invalid XML. Some choice quotes:

The only reason I can imagine you might choose a microformat over a macroformat is because macroformats are invalid XHTML, but so what? XML doesn’t have to be valid! That’s a deliberate design decision in XML. Some say invalidity is the real revolution in XML. It’s what XML brings to the table that SGML never had.

Microformats bring exactly nothing to the table. All they do is complexify the markup and make it far harder to address with XPath and other XML tools.

The strict valid-XML-with-a-schema folks remind of the strict strong-typing folks. In my experience, in general neither idea is worth the trouble it brings.