A few days ago my MacBook Pro asked me whether I'd like to update Mac OS X from 10.4.8 to version 10.4.9. I said 'yes', hoping to see some noticable improvements in speed or stability. However, instead my system seems to have become slightly less stable, actually requiring a reboot today. Several programs crashed.
One other thing I noted is that the quick-shutdown key-combo (Ctrl-Alt-Cmd-Eject) does not work anymore. That is, until I read on a Dutch forum that you need to keep the Eject down for a bit longer. Then it does work.
Perhaps the stability degradation is a clever way to force people into buying Leopard :-)