The huge system update to Mac OS X 10.5.3 with 200 to over 400 MB resolves a lot of bugs and adds up some additional features to Leopard. Example: More digital cameras with their specific RAW-formats are supported now. Or the possibility to sync the Apple Address Book application with Google Contacts.
Great to see that WIFI stability was increased by the Apple developers.
An annoying - but resolvable - problem appeared after updating when I tried to use the time logging software TimeLog 4: All log entries within TimeLog are saved in iCal - and after upgrading to 10.5.3 my work time entries just doubled. It’s easy to fix the problem by deleting every dublicate log entry - but you better checkt the timesheet in detail before you hand it out to your customer

The new Quicktime and iTunes update is available via Software Update. But be careful: Some users has serious problems after the update installation.
This time the update bug catched me, too: After finishing the update of iTunes 7.1 and Quicktime 7.1.5 via Software Update the Finder crashed every 90 seconds and restarted. Restarting the Finder brings all Finder-windows in the foreground - even if you are working in an other program … Steve would say … "Yuck!"
In my case, the StuffIt-Engine of StuffIt Deluxe was the problem - thanks to the glorious bug tracker Robert from the Apple-support discussion forum.
So this means wait, see and hope that Apple will fix that conflict - or update Quicktime, iTunes and StuffIt Deluxe.

Links: Apple-Support discussion forum