To what extent is MariaDB Java client a perfect replacement for MySQL JDBC (Connector/J)?
I'm getting a bit nervous.

First, the upside: I've used the MariaDB JDBC driver in an "always on" service in production with a database connection pool and it is running fine for a couple of months now. The service only uses basic JDBC functions with simple queries (e.g. no joins, no blobs). I'm about to release another "always on" service that uses the MariaDB JDBC driver in a similar fashion.
I decided to use the MariaDB JDBC driver after (re)viewing the source code: compared to the Oracle MySQL JDBC driver source code the the MariaDB JDBC driver source code is a lot more readable and understandable.

But I get the impression (9/2014) development has slowed down (link is to a graph showing bugs created vs. resolved). If I browse through the open bugs I see many serious bugs open for far too long (issues with Hibernate and also time-zones/dates). Some of these may not be bugs but then they should have been closed by now. I also encountered one bug (via STASH-4012) that should be re-opened: bug CONJ-72 is closed but the last comment (from 4/2014) indicates the problem is not resolved for Hibernate. 

All in all I do not consider the MariaDB JDBC driver a perfect replacement for the Oracle MySQL JDBC driver. I'm happy to continue to use it for the simple stuff (where all queries are known beforehand and tested) for now, but I might have to switch back to the Oracle MySQL JDBC driver in the future.