Apple released MacOS X Version 10.5.3 earlier today. I first learned about the release when I was checking out Summize for the first time and saw references to Mac OS X and Leopard on the home page as "trending topics".
MacWorld has a good summary of the important fixes and new features included in the update. However, I performed the update before the Macworld article came out. I made the decision to take the update after I read through the About the Mac OS X 10.5.3 Update article in the Apple Knowledge Base.
Whenever a software developer like Apple releases a major update, I look at the list of fixes to see if any of them are must-haves for me. The fix that caused me to start the update process immediately related to Spaces:
Resolves an issue in which switching to a different space and returning back to the original space may reorder the application windows with a different active window.
This bug was a significant productivity drain for me. I often switch back and forth between the space where I keep Mail.app and the space where I do my blog writing or the space where I do web development. Every time I returned to a space, the active window changed from the one I had last used. I usually had to invoke Exposé to find the window I needed to use.There are many, many fixes and new features besides this one. The new feature that surprised me the most was the Google contact sync capability for iPhone users. I've seen a couple of reports that this feature isn't flawless, but I'd love it if Apple and Google got together and made iPhone / iTunes sync happen for both Gmail and Google Calendar.