I ordered a family pack from Amazon on Sunday, received it in the mail Tuesday afternoon and promptly backed-up and did a format/install on my MacBook as soon as I got home from work.
Stacks is kinda cool, but I don't care for the way it changes the icon - I'd rather it be just the folder icon and stay that way.
I'm sure Spaces will be great on my MacBook once I get it set-up.
I have other methods of backing-up so I probably won't even use Time Machine.
I LOVE having my Shared computers/files LISTED in the Finder!! Coupled with the "Connect As" button this feature makes using files from other computers much easier.
As others have mentioned, Cover Flow is nice for browsing photos/wallpapers, but I think it will have a somewhat limited use for other types of files (at least on a 13" screen). I'm not a big fan of Cover Flow in iTunes -- I guess I don't really get too worked up about what the album cover looks like, but I do like it for browsing photos. Now if Apple would let you right-click/option-click a wallpaper to set is as a background rather than having to go all the way into preferences! That's about the only thing I miss from Windows.
Transparent menu bar. Still undecided, but I think it will present some problems for folks who like to use the menu bar to display menulets like MenuMeters. I had some difficulty seeing the graphics with the wallpaper I was using last night.
The brushed look of Tiger is better than the flat gray (IMO).
I'm still underwhelmed by Safari (I know, blasphemous, huh?). I'd been using the Beta 3 on Tiger some of the time but for some reason it just feels slower at rendering the pages than Firefox and even Camino. The final version on Leopard doesn't feel any different.
Can't wait to "play around" with all of the other things Leopard has to offer.
I will wait to upgrade my Mac Pro until the apps I use are updated for Leopard.