Well, I bravely updated our home iMac G5 to Mac OS X 10.5.3 today:
I say bravely, because when I updated my PowerBook G4, restarting was a bit problematic. I had the spinning charcoal dashes for easily 15 minutes. Finally gave up, started again, gave up again and restarted again, and was trying to go into single-user mode, but fortunately was just a tad late with my Command-S. Third time was the charm, and it started up fine.
But the iMac G5 was routine. Install, restart, continue. No problems at all (such as those mentioned at Macintouch’s 10.4.3 report).