For everyone who has upgraded to a 5.1.x version. This is mostly a bug fix release and it is up to you if you want to upgrade. There are no apparent security issues solved in this release.
Check all the fixes and changes that are listed on the release notes to see what issues are resolved and if you are affected. Most of them are concerning replication.
I’ve compiled this version and did some simple tests on my test servers and it worked without any problems.
Read the documentation on how to ugrade MySQL.