It's a fork of MySQL that provides substantially better durability, admin tools, an easy hot-backup solution, better instrumentation of your DB, etc etc.

Deploying is it a question of "stop db, replace mysql server binary with percona server binary, start db".

My personal opinion is that unless you have a support contract with MySQL AB, there is very little reason not to use Percona.

Performance, for one: http://www.percona.com/software/percona-server/benchmarks/

They also provide a lot of nice tools: http://www.percona.com/software/percona-toolkit/

Percona also absorbed Maatkit in their tools for those familiar with it. Very handy stuff in there for automation of various processes.

Pretty happy with Percona as a drop in replacement for MySQL.

Their Xtradb storage engine has very tangible performance benefits over InnoDB, especially on SSDs.

Beyond the other listed benefits: Percona also provides support contracts quite inexpensively, which can be worth their weight in diamonds. I bought a support contract after our Percona instance started to slow down, and the level of quality of the analysis and the results blew my mind. And I've been doing SQL for 25 years.