The KDE 4.2 desktop used in Fedora 11 and above is significantly different from the KDE 3.5 desktop used in Fedora 8 and earlier. The elements, however should all be familiar and it serves the same purpose; to allow the you the user to interact with the compute by allowing you to launch and interact with the application programs that you use to accomplish work.
KDE 4.0 was released into the wild for the first time with a distribution in Fedora 9. There have been huge changes to KDE 4.0 over previous versions and it was still suffering from the dreaded dot-zero release diseases until Version 4.2. It was not ready for prime-time prior to the 4.2 release and even the KDE developers had been telling us this.
It is now a killer desktop and I expect more interesting developments in the future. You should skip Fedora 9 or use GNOME. KDE 4.0 is only for those geeks who enjoy self-flagellation. Read my review, Fedora 9 Not Ready for Most Users. KDE 4.2.4 is ready and you should feel comfortable using it in Fedora 10 or 11 and beyond, and in Kubuntu 9.04.
In March of 2013, Fedora 18 is the current release and KDE is at 4.9.5 and has become a very powerful and flexible, mature desktop. I still test out other desktops from time to time, but always come back to KDE, primarily because of its nearly infinite flexibility.