The quality of MySQL 5.4.0 is very high for a beta product.
Four weeks after we released it as beta we have not had
any real serious bugs reported yet. There are some issues
due to deprecation of features, version numbers and a
bug in the SHOW INNODB STATUS printout and some concerns
with the new defaults when running on low-end machines.
It's also important as usual to read the documentation
before upgrading, it contains some instructions needed to
make an upgrade successful. The upgrade issue comes from
changing the defaults of the InnoDB log file sizes.
For those of you who want to know more about MySQL 5.4.0
and it's characteristics and why you should use it, please
join this webinar where Allan Packer will explain what
has been done in MySQL 5.4.0.