I've used Leopard all weekend and I am hooked. I expected it to be just a mediocre upgrade without those gotcha features. I've been proven totally wrong. Since top ten lists are all the rage with the internets, here's mine.
Top ten reasons to embrace Leopard
10) I actually like the Dock now (I don't hide it in the basement between uses anymore)
9) Coverflow sucks less than I thought (It is actually handy for folders with images, but not much else)
8) Unified UI in all applications
7) Safari 3 no longer beta (it just feels more complete now)
6) Spotlight ROCKS! (if you are using Quicksilver just for launching, go with Spotlight now)
5) Stacks is handy (much nicer than folders in the dock)
4) Todo lists in mail---finally
3) Preview completely replaces Acrobat for most users
2) Time Machine actually works (if you need it)
1) It's FAST AS HELL (seriously, my Mac Pro finally feels like a zippy machine)
I know some people will argue that my #1 might be due to a fresh untainted system, but I've reinstalled 10.4 several times on my Mac Pro, and it never felt this fast. OS 10.5 feels solid.