For the moment I'm evaluating Fedora 12. I was a bit disappointed on the Kubuntu 9.10 release, the distribution felt a little bit slow and tiered. The only thing I'm satisfied with is the boot time, it starts quite fast.
The KDE4 desktop in Fedora feels kind of snappier than KDE in Kubuntu. I'm not sure if it's because Fedora is compiled for i686 and not i386 or if the team have done any optimizations. I didn't enabled desktop effects in neither distribution. I installed the nvidia proprietary driver in Kubuntu but use Nouveau (open source driver) in Fedora. I might try the proprietary driver in Fedora to see if I gain any speed.
There's one bad thing I found in Fedora in comparision with Kubuntu, KPackageKit. It takes ages before starting compared to the Kubuntu version so I'm using yum from the shell prompt instead. Well, to be honest, I'm not a heavy user of UI front-ends to package management systems. In Kubuntu I prefer using aptitude instead of KPackageKit.
I think I'll give Fedora a chance and try it out for a while.