Tim Bray - Choose RELAX Now:

"Everybody who actually touches the technology has known the truth for years, and it’s time to stop sweeping it under the rug. W3C XML Schemas (XSD) suck. They are hard to read, hard to write, hard to understand, have interoperability problems, and are unable to describe lots of things you want to do all the time in XML. Schemas based on Relax NG, also known as ISO Standard 19757, are easy to write, easy to read, are backed by a rigorous formalism for interoperability, and can describe immensely more different XML constructs."