The ability to sync your Address Book contacts with Gmail - one of 10.5.3's most useful new features - could have used a bit more real world testing. Rather than just syncing your beautiful, groomed, deliberately added contacts like any reasonable person would expect, Address Book downloads all of your Gmail account's automatically collected addresses as well, which is to say pretty much everyone you've ever emailed, ever.
Despite this Rolodex-wrecking flaw, Leopard's update introduced plenty of other features that actually make the operating system better. Spaces now behaves like a proper virtual desktop system, and Time Machine is significantly more reliable, as well as playing nicer with Time Capsule. Mail, iChat, Airport and Automator also underwent minor refinements. Taking in to consideration these enhancements as well as a number of critical security fixes, 10.5.3 is still a recommended download in spite of the Address Book goof. For those who just had to click every new radio button in 10.5.3, the same helpful reader who sent in the Gmail sync warning has posted a quick fix here.