There are a number of significant new features in MySQL 5.5 including semi-synchronous replication, SIGNAL and RESIGNAL, the PERFORMANCE_SCHEMA, additional STATUS variables, new partitioning options, different default storage engine, better UTF8 support and removal of deprecated functions just to list key considerations.
However some of the performance improvements are worth the investment of time. For a high concurrency InnoDB environment one new configuration alone can provide a 50% improvement in performance. Other settings can help in the reduction of downward spikes in performance due to internal InnoDB disk management. Some new configuration settings are off by default. See the actual benefits of these new features and what you should be using to maximize your database performance.