A lot of people, don't think that XML, is that good, but it's really great, XMl doesn't do anything special, it just tells the computer what does a web page has, for example, your computer will never now that this page is a discussion about xml, but with XML, it will now, and it's great for RSS fed, like it's been said
XML is useful now, but will be more useful in the future. Once more things are marked with it and support for it is more universal. Personally I think it's an ellegent system for classifying data.
People usually don't know what XML can do.
Besides RSS feeds and creating flatfile databases, XML is also a marvelous tool to work with Flash; for example, PHP+Flash chat is made easy with XML.
same xml file can be used in large number of programming languages, like php, vb.net, asp and a lot of others. Using xml, u can easily transfer data prom one program to other