Nice move. I find MariaDB better than MySQL in any way. Small things like statistics, user prompt makes it more user friendly.
Performance is better. With big data set you don't need benchmark, you can see it.
Replacement of MyISAM with crash safe Aria engine, for those who think MyISAM is quicker.
nicely optimized Percona-XtraDB replacement of InnoDB engine.
Build in Sphinx.
better replication performance.
some extra information in the binary and slow logs.
Plenty of new features coming in next MariaDB 10 release like multi-source replication and full text search in InnoDB engine.
When I joined our company, the first thing I did replaced all MySQL 5.0 - 5.1 databases with MariaDB 5.5.
The process of migrating is painless. few things to remember: pay attention to my.cnf file. better use MariaDB template and put what ever needs in it. move origial my.cnf out of the way. The same about mysql.d config folders.