Recently upgraded from MySQL 5.1 -> MariaDB 5.5 with no issues what so ever. We have ~90 tables where the largest table has 40,000,000 entries.

I configured the new database servers as slaves of the old ones. This enabled me to verify things in production and do a safe upgrade. Promoting the MariaDB slaves to master was the only required action in terms of switching over. No code changes were required in our application code.