I've got a prototype eXistDB server, built as a WebObjects application, running on an iMac on my desk. Works pretty well, and it does some great XQuery stuff. I've entered the CAREO metadata, current as of a week ago, so it's got 3733 IMS LOM records to play with.
It has a simple search (just enter a term and hit the button), as well as a generic XQuery entry panel. Feel free to experiment on any XQuery statements you want (do a simple search and look at the source HTML for the result page for a starting point...) Go ahead and try some funky boolean searches like "earth image satellite". Still some refinements left to go (like limits on search results - it's currently possible to return the entire database as a result of a query - not recommended). I also have to play around with handling multiple schema - IMS LOM, DublinCore, MPEG7, IMS CP, METS, ... for both querying and retrieval.
Basically, eXist is pretty darned good, and Wolfgang (the developer) is quite responsive. Not sure how it compares to XStreamDB performance-wise, but it does beat it on the cost...