I develop a Rails 3 web app and recently moved from MySQL to MariaDB. Well actually the guys at Rails Machine did the move for me. I don't know what all they did on their end, so can't speak much for the server admin side. We ended up making the switch since I was moving to a high availability setup with multiple DB servers.

On the web app side, I am using the mysql2 gem and made zero changes to my code. Also, I frequently import SQL dumps from MariaDB into my local MySQL development with no problems.

The only issue I had was related to some nasty SQL I had in a select in a finder (yeah I know bad practice). MySQL was returning a 0 while MariaDB returned NULL. Either way, it was my fault for messy code and when I fixed the select, everything was good.

There's one silly difference I noticed. Timestamps in mariadb dumps have their size appended. Mysql doesn't like that, so you need to filter them out. Otherwise pretty much a drop-in.

I maintain a reasonably complex web app in production that uses stored procedures extensively. I am running it in production with both MariaDB 5.5.x and MySQL 5.5.x.

I have not experienced any compatibility issues with MariaDB vs. MySQL.

Seamless. I'm not using stored procedures, views, etc., but for some simple InnoDB databases, MariaDB was a drop-in replacement. The command line tools are even still called mysql.

We were thinking about it. I fired up our apps testsuite and found a bug: https://mariadb.atlassian.net/browse/MDEV-4711

The bug was now been analyzed but not fixed for ~3 releases.

That said I also ran the testsuite against 5.6 and found some problems, too. You might also be concerned about http://bugs.mysql.com/bug.php?id=69274 - this means that if you loose an idb file during a crash you're out of luck to use your db ever again.

We're thus currently stuck at percona server 5.5 with regular investigation of the alternatives.

We're currently doing one. It's been relatively painless, most of the difficulty being simultaneously moving off old myisam tables to innodb -> xtradb

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.