Almost everything worth saying about Mac OS X Leopard has already been written by people you can find via Daring Fireball, including John Gruber himself and an assortment of others via a number of linked posts over the past few months (especially John Siracusa).
I love John Siracusa- he has a way of exploring and explaining OS internals that gets right to the heart of my appreciation for computers. Beneath the surface of an operating system lies a complicated web of systems, technologies, rules, and exceptions, and the logic behind it all and the functionality exposed as a result fascinates me.
My friend Jason also has some choice words about Leopard.
My bottom line: the underlying operating system continues to make impressive strides forward with regard to architecture and core technologies. Siracusa's review provides some great illumination along these lines.
Time Machine is clever- goofy, but clever- and Back to My Mac is inspired. Quick Look and better icon previews are nice additions. Spaces seems nicely implemented but not a natural fit for my workflow. The numerous tweaks and nudges to virtually every aspect of the system are mostly for the better.
So far, so good.
The new Dock sucks- it's ugly, harder to look at, and Stacks don't work well at all. Siracusa nails most of the reasons why, but I'd like to add one more: you can no longer drag the Applications folder into the Dock and use it as an application launcher. Previously, clicking and holding on a folder would reveal a list of all of its contents, any of which could be clicked to open. The list would scroll if its contents were longer than the height of the screen. Ergo, dragging the Applications folder into the Dock provided a really elegant way to get a full list of all of your Applications with one click.
With Leopard, my Applications folder in the Dock now opens as a grid Stack, which displays 62 icons and then an option to open the folder as a window in the Finder to reveal the additional 18 items I have in my Applications folder. Which renders it completely useless as an application launcher.
This could be easily remedied by adding a "View as list" option to the context menu for individual Stacks, which would fix that problem. (Siracusa similarly asks for a "View as window" option, but I think list is even more sorely missed).
In addition to the problems with the Dock, there are UI and usability issues with the new translucent menu bar and Leopard's folder icons- all described well by others already mentioned.
When taken as a whole, these issues are troubling because they demonstrate that Apple's traditionally meticulous attention to detail, form, and function has been thrown for a loop with Leopard. In that regard, these aspects of Leopard are not "Mac-like", and for a Mac OS X upgrade, that's an odd situation to be in.
The big picture, however, is mostly positive. Hopefully these visual and functional UI issues will be addressed in subsequent releases, and assuming they are, there's no doubt that Leopard is a solid step forward for OS X on all counts.