Derek (no last name) is a lead developer at Microsoft on their XML libraries (first MSXML and now System.Xml). He's just started a blog, and he seems to know what he's talking about. His post Where XML Goes Astray correctly hits some of the dark corners of XML. He especially gets right the difference between documents and data, and understands that XML was fundamentally aimed at documents, which is what makes data difficult sometimes.
The problem with XML is the M. The X and the L are just fine. Remember, the M stands for Markup, which means adding meta-information (tags) in and around a body of text.