This AM I upgraded my two macs, a first generation MBP and a G5 iMac, to Leopard 10.5.3. Please note, the upgrade is large, at 450 MB, and when you finally restart it seems to cause Mail and iCal to freeze. You need to allow it to perform its processing on those apps, because it apparently does some kind of check on the mail and calendar contents.
After the upgrade, I noticed that Option-Cmd-Eject (puts OS X to “sleep”) works more smoothly, and that opening the laptop from the sleep state is quicker. Perhaps that is a coincidence, but this update is said to have covered a large number of bugs.