CentOS 5.4 was officially released yesterday (Oct 21, 2009) and is available as a full download, or as an upgrade via yum. Release notes can be found here: http://wiki.centos.org/Manuals/ReleaseNotes/CentOS5.4.
My personal experience so far is good; I have upgraded all 3 of my slices and have not yet seen any issues. However, I can't guarantee that you will experience the same! Be sure to read the release notes to see what's changed before charging gung-ho into an upgrade!