I have installed openSUSE factory (our developement version) together with the KDE 4 desktop on my Laptop and gave it a quick test this week: The desktop looks much cleaner and polished, the new icons and the oxygen theme have a fresh look. Gwenview - the image viewer - has been greatly reworked and streamlined. The desktop gave audible feedback for many actions - something I would personally rather disable but others might like it.
The System Configuration has been redone and the modules look really nice. Changing underlying libraries does not always give visible results - but in the case of KDE4, you can see that the new libraries and frameworks have been rewritten and enabled the developers to write better applications and tools.
The "Zoom out" feature confused me, I don't know what the real benefit of it is. I also hit some bugs, e.g. the desktop effects did not work for me directly (I'm glad that this is so nicely integrated and finally easy to setup). I also missed a few features, e.g. the panel is a bit too large for me and I have not found a way to create a smaller version of it.
I suggest to look also at the KDE 4.0 Visual Guide which provides a quick overview of various new and improved KDE 4.0 technologies.
Not all packages have been ported to the new framework but you can use KDE 3 applications easily on the KDE 4 desktop. I could use the 4.0.0 release for my work without major problems.
All in all, it looks to me like a promising .0.0 release - it is the foundation for a great desktop and I look forward to see major KDE3 applications ported to KDE4!
Testing KDE 4 with openSUSE
You can install packages directly from the openSUSE Build Service. On openSUSE 10.3 you can use 1-click-install to get the KDE 4.0 desktop environment:
Alternatively you can choose to install a more basic KDE 4 desktop.