Installed combo updates on both my Mac mini 2009 and my MacBook (white polycarbonate).
Mac mini installed without incident. Single reboot.
MacBook installed without incident, but hung after pressing "Restart." Forced-restart the hard way and the system rebooted twice (as with other system updates). Tried to install again to see if it was just a fluke, and it hung again after pressing "Restart." Time to hunt down which little thing is causing that, although the system update seems to have taken just fine.
I wasn't bitten by any of the bugs in 10.5.7, and there's really nothing I needed fixed, so this was all just in the name of keeping my systems up-to-date.
Apparently this update "fixes" some ATi graphics bug, but I've heard reports that people are still experiencing graphic anomalies even after applying 10.5.8. Sucks to be them!