At $work we are managing a fairly heavy MySQL instance and having scaling issues (~5TB, lots of blobs, and frequent user-managed schema changes for interactive/batch data analysis, many concurrent complex joins involving multiple 1M+ row tables) and after reviewing the various MySQL derivatives, it seems that the percona ppl are really focusing on 'real deal' operational reliability/scalability issues... (e.g. minimizing transaction locking issues for coherent backups, real-time db cloning, replication data integrity testing, etc). I strongly recommend anyone looking at mysql flavors to not overlook their offerings. Looking forward to doing some production tests of their XtraDB mysql flavor in the coming months (for now usage has been limited to unit tests and using the percona toolkit/xtrabackup)
Also: no I am not a paid sponsor.