Version 5.1 introduces several enhancements to the already rich set of features. Most notable are partitioning, row based replication, the event scheduler, a new plugin infrastructure, and logs on demand.
There are more new features, but the general improvement is better performance and manageability. Many users have already adopted MySQL 5.1 in production. Their feedback has been reported in the MySQL technical articles. Check them out. MySQL 5.1 is ready for prime time!