I've just installed it and I must say it is very impressive. It looks nice and is easy to use. It is just the sort of tool I've been looking for.

XML Spy is a professional validating XML editor for Windows developers, IT professionals and Web-Design people that lets you edit all XML, XHTML, XSL, RDF, 3DML, and DTD files and provides three advanced views on your documents: an Enhanced Grid View for strucutred editing, a Source View with syntax-coloring for low-level work, and an integrated Browser View that support CSS and XSL style-sheets.