We have changed the release model so that instead of focusing on quality and features our release is now defined by timeliness and features. Quality is not regarded to be that important.

and

Zero critical bugs is impossible: Sure, zero bugs is desirable. And at times, we internally glorify MySQL’s past, with “production level alpha releases”. Those were the days, but those days also had less complex and less interwoven features, and fewer developers to manage.

I wonder where MySQL is headed.

So as Monty says, you are better off not upgrading to version 5.1 if you do not plan to use its new features, atleast not without testing it for a few weeks.

Links:
Oops, we did it again (MySQL 5.1 released as GA with crashing bugs)
Why You Want to Switch to MySQL 5.1
MySQL 5.1 Stability

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