I think it's worth giving a shout out to the Percona Toolkit[1] as well. It's awesome.

http://www.percona.com/doc/percona-toolkit/2.2/

Where should I use Percona Server over standard MySQL or MariaDB? It is not obvious which server provides which benefits over the others.

You should almost always use Percona over vanilla MySQL. Percona is a drop-in replacement with some tactical fixes applied. These are usually OLTP performance related or added visibility. Percona collaborates with quite a bit with the Facebook mysql team which runs an insane amount of mysql servers. As a result of lot of true operational fixes make it into Percona so you don't have to worry about them.

MariaDB is a bit more tricky. I am not sure if they are still considered a drop-in replacement like Percona. MariaDB forked off of MySQL 5.5 and ported over features from mysql 5.6 and the defunct 6.0 branches. They implemented some of those features differently than in MySQL (like GTIDs and some other replication features). If they are still considered a drop in replacement now I don't think they will be able to continue. By forking like they did they are not able to keep up with the improvements and features of the hundred(s) of mysql engineers at Oracle and I expect releases to be slow in coming from them. I was about ready to write them off completely but Google is throwing their weight behind MariaDB due to understandable political reasons so I hope that will save MariaDB. In any result I think MariaDB will diverge farther and farther from MySQL.

My plan is to use Percona 5.6 until MariaDB 10 is GA

Percona contributes a great deal to MariaDB, including xtradb.

Percona 5.6 is a binary replacement for MySQL 5.6 so you can just switch. They have an rpm repo and everything.

Congrats to Percona on all their hard work.

We've been using their 5.6 for a few weeks now, works great.

ps. please consider backporting the SELECT COUNT(*) improvements that were added to innodb in mysql 5.7