I hope one day I write a similar post for HINTs, an optimizer world which always guarantees the *best* path for your each query without any additional interrupts..
[from metalink's 11g Upgrade Companion]
Oracle highly recommends the use of SQL plan baselines instead of the stored outlines after 11gR1. With Oracle Database 11g using the SQL Plan Management (SPM) the optimizer automatically manages plans and ensures that only verified or known plans are used.
SQL Plan Management allows controlled plan evolution by only using a new plan after it has been verified to be perform better than the current plan. You should also use SQL Plan Management as part of your upgrade strategy. Please see the best practices section for more information.
[/from metalink's 11g Upgrade Companion]
For more details on SQL Plan Management