I saw Mac OS X 10.3.9 appear in Software Update yesterday, about 27 seconds before I saw it mentioned on MacInTouch and right about the time several AIM and iChat buddies let me know it had been released.
I decided as usual to wait a day or two before installing the 10.3.9 update, partly to see what problems other users encountered and partly because the OS X 10.4 "Tiger" release is less than two weeks away and I didn't feel a pressing need to do two upgrades in such a short time. But I relented today and backed up my files before letting Software Update go to town on my PowerBook 12-inch.
It went very smoothly, and now after a few hours I've noticed only one problem. In Safari, I'd previously been able to go to most sites just by typing their names without the www. prefix or the .com or other top-level-domain endings; Safari would automatically guess the full URL and take me to the site in a matter of seconds.
Now, however, I'm seeing the endless spinning-wheel-of-death and my CPU gets pegged at full usage while Safari goes completely non-responsive, and I end up having to force-quit to get anything working.
A minor annoyance, such that I haven't bothered troubleshooting it beyond making sure it's predictably reproducible. And it's easily enough fixed: I just have to remember not to try the shortcut-to-URL trick for the time being.
MacInTouch has more info about the updates and readers' initial reports of experiences on its main page (I'll update my link as the MacInTouch content location changes). I'd recommend checking those reports out before you install, and as always, make sure you have a good (recent!) backup of your important data before you undertake any major OS update.