Mac OS X Leopard
After a few days with Leopard on my new MacBook, I absolutely love the new interface. Initially, I didn't think that the “cover flow” method of browsing files would be something that I would use, but it actually helps a lot when you have a large number of similar files in a folder.
The most important change, at least for me, is the new speed that “spotlight” seems to have found. Otherwise, most of the changes appear to be under the hood, and the Apple 3rd party developer community haven't really found a way to take advantage yet.
As far as speed is concerned, I can tell you that on my MacBook, it generally feels fast. Rosetta applications actually feel a little slower, but that is probably subjective. The UI feels faster than Tiger, but if so, it is definitely marginal.
John Siracusa has done an incredible in-depth review, with which I can find no fault. Basically I was going to write something like this, but he has done a much better job, so you could probably read his: