I have not yet seen enough of 5.1 in real-world production environments.
MySQL 5.1 rease as “GA” seems to be the most controversial to date. It had very negative response from Monty, original MySQL Founder and controversial responses in community.
Time will tell.
From an article referenced below:
As a Summary I think this was inevitable MySQL 5.1 will be released for commercial/marketing reasons, not because it really meets quality guidelines. At the same time I would like to send my warm congratulations to MySQL 5.1 development team which made this release better quality than MySQL 5.0 initial release.
Reference:
http://www.mysqlperformanceblog.com/2008/12/10/mysql-51-went-to-ga-was-it-a-good-move/