We're using MariaDB at linkup.com and love it! Whenever I read "MySQL is becoming closed source" I think its just an indication that MariaDB needs more community interaction (like how the Fedora team ramped up marketing efforts when RedHat killed their non-RHEL distro.)
When I got my new Macbook Air I decided to give mariadb a try locally instead of mysql. So far so good! 'brew install mariadb' and you're off to the races. It's 100% binary compatible, as far as I know.
So far working great with some local wordpress/php apps as well as older django apps.
I'm migrating slowly but surely to PostgreSQL though.