Well I’ve made the switch from Tiger to Leopard, and I can say I love it. Time Machine is awesome, the switch from a 32 bit OS to a 64 bit one makes every thin run real fast, plus a whole bunch of other improvements. The only casualties so far were a couple of beloved mail plugins, Letterbox and HTTPMAIL. At least I know Letterbox will be updated for 10.5 soon.
Stacks doesn’t do much for me, but I like the “glass shelf” appearance of the dock. Spaces has already become very useful for me, and the combination of “quick look”, “cover flow” and a much speedier Spotlight (now with boolean operators & saved searches), makes finding files a pleasure. Many pundits have criticized the transparency of the menu bar, but I like the aesthetics of it.
iCal has seen some nice improvements, iChat is a blast (now with much better audio clarity), and I really like what they’ve done with the new Safari with a cool find feature and the ability to “clip” a portion of a webpage and turn it into a widget. Mail, Preview, Front Row, and DVD Player have all seen improvements. The built in Dictionary now supports entries from Wikipedia. If you use Photo Booth, Parental Controls, Image Capture, VPN, Terminal, or just about any other feature you can think of in Mac OS X, you’ll find at least some changes. So are 300-plus new features worth $129? Yes, and more!