Jon Udell published an introduction to E4X a while back. I've already been very interested in E4X, and I truly believe that this technology fills a gap that hasn't been really catered for so far: simply querying XML. The DOM XPath APIs are pretty heavy to work with and DOM Core isn't truly designed to be used as a query language. I think Jon Udell has is spot on when concluding to his article when he says that the idea is to provide an 80/20 solution that does what most people need in a way that most people can easily understand. I think these APIs are particularly useful in a world where web developers are relying more and more on mechanisms such as XmlHttpRequest and getURL(), and I really can't wait to see SVG and CDF implementations to offer E4X to developers.