I must say that OS X Leopard is one of the best products Apple has ever created. Some say looks and computing should never interfere with each other, but if you're trying to sell your product, nobody would buy something Windows 1.0-ish today. I think Apple was right when they made the menu bar transparent and the Dock reflective. Compared to a Tiger desktop, the new Leopard one makes the screen seem larger and more spacious (pun intended with the "Spaces" feature).
Spotlight is now no longer a novelty. Now, it searches web history, dictionary, calculator, remote computers, and even supports Boolean searches.
Leopard is now environmentally friendly - MUCH more than either the old AppleWorks or the current MS Office:mac 2004 box, which happen to be the same size. Ubuntu still beats it, though :P (environmentally, of course)
Now, if I ever wanted to do some simple chroma keying for a movie, I'd have to get on Linux, run Cinelerra, and import my new file back into iMovie. No more is this the case. With the new Photo Booth, you can not only capture images, but also video. You can also add any video or photo as a background, so Linux might be left alone for a while.
Stacks are a really good feature, in my opinion. I have instant access to my documents/downloads and are perfect when I'm working on a big programming project (although it's quite small). I'm working on a Java application now, and with the 10 files my "big" project, Stacks lets me easily access them without having to open a Finder window.
Time Machine is an awesome new feature. If you say "Oh, but Apple copied it from Microsoft," you're wrong. Automatic backups, yes, have been part of Windows for a while now, but never was Microsoft able to make it look nice. And yes, you *can* make it work on a single hard disk. With disk utility, partitioning your HD is quick and easy, and you can easily add and remove partitions on the fly.
So you can see that Leopard is a very nice operating system. Much better than Vista, in my opinion, and a great evolution ahead of Tiger. Hey, it's only $129!