Erik - My own experience with BaseX 6.0+ has been extremely positive. I needed a fast, multi-user (reading) database that implemented not just XQuery, but XQuery with standard Full Text extensions. And not for a toy database, but for a database of historical documents, initially with about 200,000 pages of text with significant internal structure.
I initially stress-tested BaseX for multiple users/queries and found a problem, which Christian Guen took seriously and rapidly fixed. (When I say stress-tested, I mean I really hammered it.) The same has been true for a few other glitches and questions.
Overall, BaseX has been very solid and very fast. It is crazy-fast for the complete indexing that it does, and it is really fast for queries. Fast enough to find tens of thousands of hits in a structured XQuery+full text search of about 200,000 book pages in under 2 seconds, including retrieval and processing of title information for the documents.
So I really like BaseX for its practical utility, and as a programmer I also appreciate that the overall approach to the system is clean and pretty rigorous. They set out to implement actual W3C standards for XQuery and XQuery with Full Text extensions, and that's what you get, plus some useful extensions.
The included simple GUI is actually very useful, not just a toy.
I place BaseX in distinct contrast to the older and apparently more established eXist system. My experience testing with it, over time as the system has evolved, has not been positive. It is cumbersome to set up - especially for indexing - and it does not follow W3C standards for XQuery Full Text but instead uses its own syntax, and if you look at the mailing list for it, it seems to have a large number of ongoing bugs. It really feels like a big, unstable hack. From a performance perspective, a much smaller subset of my full database began to seriously slow down queries.
As far the future goes, my biggest hope is that BaseX becomes better at being a true writable database system. While it appears to handle standard XQuery Update without problems (which itself is impressive!), this is restricted to one writer at a time, as I understand it. I would not try to use BaseX currently as a general purpose writable database, but I also don't need it. In my view, there is a place for XML databases alongside use of standard relational systems. In that regard, I highly recommend the H2 database system if you need a clean, powerful, fast Java relational DBMS - it is greatly superior to, e.g. Derby.
So it probably goes without saying, but I am a big fan of BaseX and I hope to see it grow in popularity and features, as it deserves.
Phil Oliver Oliver Computing LLC