Have you watched the Mac OS X Leopard guided tour yet? They show off Spaces, Quick Look, Time Machine and other new and improved features.
Now I do 99.9% of my email via Gmail (and via the web interface, which shocks me) I’ve been thinking of switching to Apple Mail for the other 0.1%. The new features in Apple Mail looked really cool, especially the integration with iCal and the other Mac apps. When I do the wipe and install for Leopard I’ll make the switch.
I was impressed that at the end of the guided tour video they even showed how to upgrade. I have friends who would very much welcome that demo.