So, I got my copy of Leopard on Friday and installed it Friday night. There are definitely a few cool things about it... lets do it in list form.
Cool: it has virtual desktops built in.
uncool: there aren't enough function keys on my laptop to map the virtual desktops. (This isn't a problem at home since there aren't dedicated keys for keyboard backlight brightness and screen mirroring.)
Cool: seems to be rock solid, no crashes or weirdness.
Uncool: the menu bar is now a light shade of blue.
Cool: you can now set mail to permanently accept certificates from servers with self-signed certs. (cough, dactyl.)
cool: time machine
uncool: the fact that time machine will only really work correctly with an external disk.
Basically, it seems to work really well. However, it doesn't do that much that's new. Time machine is probably the killer reason to upgrade, but since most of us are using laptops the usefulness is a bit diminished.
So here are a few guidelines:
You might as well upgrade now. It works and it's only going to get better.
Before you install, make sure you have a backup and you've let software update update everything that it can.
Use the "archive and install" update method. This will make sure that any weird little programs you installed a year and a half ago won't cause problems for you now.
Make sure you have as much memory as your computer can handle. (This isn't a Leopard-specific thing, but more memory will always make your computer faster.)
Read http://www.macosxhints.com to find out all the random cool stuff once people discover it.
In other recent-ish Apple news, you should really look at the new version of iWork. Keynote is about a million times better than powerpoint and the new version is pretty nice.
(Oh yeah, Leopard requires at least a 1 Ghz G4. So your computer has to be older than mine to have any problems. I can't think of any of you using older hardware. Maybe Brian.)