I finally got Leopard, Mac OS X 10.5, installed yesterday. So far I've got to say its nice. I started the install yesterday while we were prepping for Saturday night worship, and it was up and running before the service was over.
I really like the UI updates (except for the mirror dock). Stacks are great. I've consolidated a lot of my applications into stacks to clean up the dock – a stack for CS3 Master Collection, Final Cut Studio, iWork, Office. Other nice features, when doing a screen shot of a region the crosshair shows a tooltip of x/y coordinates and then clipping dimensions.
Last night I hooked up a drive for Time Machine. It took most of the night to do a backup, but the UI is gorgeous. It's not the most functional backup tool I've ever used, but it's easy. Ultimately when it comes to back ups, easy is WAY more important than a powerful interface, because if it's easy people will actually use it. Time Machine is brain-dead simple.
Spotlight is much improved. It's faster, includes dictionary definitions, does math like Google (which lets me take Calculator out of my dock). The speed improvement is much needed, and greatly appreciated.
While the CoverFlow view in the Finder is nice it's a bit silly in my opinion. Quick Look on the other hand is great – it lets you preview documents without opening the main application. This is great for Office documents, PDF files, and other files that might be used for quick reference. I'm already using Quick Look and liking it. The Finder has other UI improvements that make it a joy to use – a better sidebar, a breadcrumb like path view,
I haven't had a chance to use the iChat screen sharing yet, but I'm very excited about this feature. I think that there will be countless uses for remote desktop collaboration. It will be useful for training, for quick fixes without having to walk across the room. It will dramatically improve collaboration.
My favorite feature of Leopard is Spaces. It's essentially lets you set up multiple desktops on your computer. I intend to have a space for video editing, general web/email, miscellaneous documents, and Parallels. I was really pleased to discover that Parallels will stay in full-screen within a Space. This way I can have a Space dedicated to full-screen Windows (I know it's silly, but I use Windows only apps almost every day). Spaces makes it easy to have a lot of applications running and not have your screen be a jumble of windows.
I'm sure I'll come across many more features of Leopard that I like, but these are the ones that win out in a few hours of use.