I'm running MySQL for most of our DBs right now, and would like to get off the Oracle ship before it takes more of a turn. My understanding is that the transition from MySQL->MariaDB should be trivial, but going the other way is not so easy. Has anyone made this jump yet?
Edit: Bonus points if you have anecdotal information on doing this with Debian/Ubuntu.
I have migrated various servers from MySQL to MariaDB a few weeks ago without any problem.
If you want the full list of what might not be compatible, you can have a look here: http://kb.askmonty.org/en/mariadb-versus-mysql-compatibility.
If you're upgrading to a higher version of MariaDB than MySQL was, you just have to jsut the mysql_upgrade binary as stated on http://kb.askmonty.org/en/upgrading-to-mariadb-from-mysql.
Moreover, with MariaDB 5.3 (released at the end of July), they now provide a proper repository, so there's no need to install by hand : http://downloads.askmonty.org/mariadb/repositories/
I know of a friend who moved his vBulletin forum to MariaDB using the XtraDB engine without problems and noted some performance improvements compared to MySQL with InnoDB.
One of the things to take note of is that NDB does not work on MariaDB[Source].
After reading http://vbtechsupport.com/606/, I migrated most of my servers over to MariaDB 5.2 builds and alot of clients have done the same with positive gains. Definitely worth it!
Same here - migrated our only MySQL database to MariaDB two years ago. It was as easy as removing MySQL server package and installing MariaDB instead.