What is Percona?
How does it differ from MySQL?
When should we consider switching (or upgrading) from stock MySQL to Percona?
To add some specifics in our situation we almost exclusively use InnoDB (which I understand Percona has done a lot of optimizing for) with extensive foreign key constraints and a few stored procedures.
What we are finding at present is that MySQL is poorly optimizing our queries and so any query that goes above 3-4 joins we have to build explicitly with STRAIGHT joins to improve performance.