I finally decided to switch to Mac OS X 10.5, aka Leopard, two days ago. It's impressive. Apart from the new UI (which was not my primary criteria), Leopard comes with plenty of updates and new functionalities. To name a few:
From the daily user perspective, it should be noteworthy that Mac OS X now includes three additional components:
and the Finder has been improved.
What else? The Spotlight indexing and searching crawler is now fully integrated into the Finder, which becomes itself the core interface for navigating through the file system (although I still prefer the command line). The whole icon design has been redrawn, keeping the organization of the file system unaffected. Fink is as easier to install as before and provides several add-ons, especially those open-source free software that I like: asciidoc, graphviz graphviz, gnuplot, subversion, etc.