I’ve just finished updating all my Mac’s with the security update 2008-005. The update went without any problem and my setup was not affected in a negative way. Every item had started after the reboot and worked according to expectations.
If you want to read more on what is affected in the updates I would suggest reading the Apple support site for the 2008-005 Security update.
In case of doubt, please make a full bootable disk image backup with Cabon Copy Cloner.
Update: It is always good to wait a few days and monitor the blogosphere and forums to see if there are any major issues with the last update. It’s good to be prepared!