From all of the videos and demos that I've seen of Leopard online, it looks very impressive. The new Time Machine with automatic file backup onto an external hard drive looks like a great feature, as well as Coverflow and "live" thumbnail images for every file in the Finder.
The Stacks look to be a great space saver for the desktop and the dock, and all the new features and cross-integration between Mail, iCal, iLife, Chat, and the rest of the Mac programs looks amazing.
And while I may not be standing in line to get Leopard tonight (my G4 Mac Mini is barely above the minimum system requirements, and my PowerMac G4 is still running Panther*), I am planning on getting a new iMac within the next 3-6 months, and dual booting it into Leopard and XP Pro. (My Dell laptop doesn't take too kindly to intense CAD work.) And since Leopard is now fully UNIX-compliant, I'm hoping that I'll now be able to run a UNIX workstation version of Pro/ENGINEER through x11 natively in Leopard (SSHing into their UNIX terminal server works too), without having to dual boot. I'm keeping my fingers crossed there.
Now while I will wait until I get to actually use Leopard for a while before I make a final decision, I'm cautiously optimistic that it certainly does look like Leopard will be a much more powerful [and IMHO better] operating system than Windows Vista.
* I'm planning to upgrade the RAM, install several USB2/Firewire PCI cards in it, and plug in several hard drives to use as a network file server at my home. It's just too beautiful of a computer tower to throw away/recycle.