I tend to stay with stable versions of operating systems as long as possible because it keeps my level of frustration low. I've been happy with Tiger but I decided to upgrade one of my Macs to Leopard recently, following the release of the huge service pack ("update") to fix instabilities in this version of the OS.
I have liked every version of OS X -- I have used them all since 10.0.0. But Leopard introduces virtual desktops ("Spaces"), which I consider to be an essential productivity feature (I love it in Linux; Windows has never had it). I also like the change in the Finder's behaviour with network shares (Samba) that enables the user to browse the shared disk without passing through the unnecessary authentication screen. These two improvements are enough to make me work more efficiently in OS X.
I don't like the default appearance of the Dock in Leopard, but the Dock offers other new, useful functionality. To those who are cautious about upgrades and appreciate being able to work smarter with a computer, I say that Leopard is now ready for your attention.