To get a feel for what an XML database can do for you, think of Bhopal or Chernobyl. Man made disasters that explode, then ruin tens of thousands of innocent lives in the process.
Actually, the concept of an XML database is excellent. You have a nearly structureless data store with effectively unlimited storage capacity. Free form search that can be made structured using XSLT and related tools. Storage of arbitrary complexity, with built in context markers to make it easy to make the managment context aware.
The problem is that I've never seen a usable commercial implementation. There are several that work fine for kilobyte to megabyte size "databases", but nothing that I can use in the real world.
At least from what I've seen, XML makes a great transport mechanism. It can get structured data from point a to point b quite effectively. It even survies "store and forward" operations quite well. But at least so far, I haven't seen anything that I'm willing to call a commercially viable XML database.