I've been testing Kubuntu with KDE4 today and have learnt two lessons.
1) There's nothing to be eager about KDE4. I mean, seriously, the guys do a good job, but this is still the same good old kde3. Those plasma and dolphin stuffs, doesn't change much, that's like a good version of the superkaramba project, it looks nice but practically doesn't change the workflow at all. And as 99% of applications still from KDE3, if don't mention internal changes, in really you've got a new nice theme for kde.
I'm little bit disappointed, the fourth version was anounced for good couple of years ago and then been delayed several times and as the result we've got something pretty raw and almost nothing new.
Yup, that's it, there's nothing in really why you should migrate to kde4 so far.
2) The quality of kubuntu is lower then the quality of ubuntu. Sorry gus, but that's true. There are several things which works out of box in ubuntu, but doesn't work in kubuntu. Several things are left behind in order to keep the kde-purity, that left a bad feeling.