Apple has released to fix a wide variety of problems in key Leopard features and to block numerous security vulnerabilities (see " ," 2008-05-29). Although Apple does provide more detail about what was changed than is normal for the company, the release notes are still rather terse and seldom give any indication of the severity or frequency of the bugs that were fixed.
That said, it strikes me as important that 10.5.3:
But honestly, the fixes that I'm most interested in seeing are those related to wireless networking and to Time Machine, because basic network reliability is essential in today's world, and because backup software has to be rock solid or users will lose faith in it. Only two specific fixes are listed for AirPort, including improvements to 802.1X behavior and reliability, and improved reliability when using Time Capsule via AirPort. However, I've heard rumblings from users that 10.5.3 may also fix other problems that had caused network slowdowns and more.
10.5.3 includes a number of Time Machine-related fixes, including better compatibility with Time Capsule, issues related to backing up laptops running on battery power, compatibility problems with Aperture 2, reliability problems when performing a full restore, and reliable storage of Mail messages and attachments. I also gather that a problem that could cause a Time Machine sparse image file to become damaged has been fixed. In addition, the update eliminates a spurious alert message about a backup volume lacking enough free space and avoids disabling function keys after using Time Machine.
The closest I can find to a new feature in 10.5.3 is additional raw image support for several unspecified cameras; see Apple's for Aperture for the current list. Apple historically adds support for new digital cameras in operating system updates, so this isn't surprising. What is surprising is the subsequent appearance of , which adds support for new cameras such as the Canon EOS Digital Rebel XSi, a separate 2.4 MB update that appeared shortly after the 10.5.3 update.
As with any system update, we recommend making a backup of your startup hard drive before installation. When Apple applies changes to so many areas of the operating system, some incompatibilities are bound to crop up. For example, we received Apple's discussion forums, so if you're seeing crashes or other troublesome behavior, it's worth looking for fellow sufferers and possible solutions there. in Photoshop CS2 and CS3 when saving image files stored on a network volume. (According to feedback in , a workaround to the problem is to choose Save As instead of Save when saving the file.) As is always the case, there are lots of complaints about unusual problems in
Mac OS X 10.5.3 Update is available via Software Update and as both (420 MB, for updating from Mac OS 10.5.2) and  (536 MB, for updating from any previous version of Mac OS X 10.5) standalone updaters. The size you see in Software Update may vary, but will likely be smaller; my MacBook reported that it needed to download only 198 MB. Note that you must have at least 1.5 GB of free space to install and optimize the update - you can usually free a lot of space in use by virtual memory swap files merely by restarting.
Shortly after the release of Mac OS X 10.5.3 Update, Apple also took the wraps off, which fixes numerous issues specific to Leopard Server. Areas seeing improvements include file services, directory services, the Active Directory Plugin, client management, collaboration services, DHCP service, DNS service, mail service, Portable Home Directory synchronization, Server Assistant, System Image Utility, and Workgroup Manager.
Mac OS X Server 10.5.3 Update is available via Software Update or as either (489 MB) or  (632 MB) standalone updates. Servers running Mac OS X Server 10.5.3 can be administered only with , which are included in Mac OS X Server 10.5.3 Update and are also available as .