I've upgraded my home server (which is also a router and an access point) from 5.2 to 5.3. All in all it was pretty flawless. Kudos to the CentOS team! Still, I'll update about few issues I've encountered:
1. Atheros wifi driver: I've been using the madwifi driver until now, which is not a part of the kernel because of licensing issues (contains proprietry firmware I guess). CentOS 5.3 introduced the ath5k driver, which is a free implementation. However, ath5k still doesn't implement "access point" mode, so I had to disable it (renamed ath5k.ko and ran depmod -a), and reinstall madwifi 0.9.4 instead (simply running make; make install).
2. udev.rules were overwritten, which cleared the changes I've made, in particular the /dev/snd permissions were reverted to CentOS default (no "audio" group, only the device owner can access audio), so I had to re-change the /dev/snd/* file permissions.