It is easy to install under ubuntu 11.10 through the backports and seems very stable so far.
Something quite important is to tweak the fonts: use Déjà Vu Sans instead of Ubuntu fonts, use RGB subpixel rendering, use Crisp desktop effects. With those settings, KDE looks very nice. It's sad that they are not default in Kubuntu.
Update March 2013: It's been a while now that it is in the standard Ubuntu repositories and I believe installed by default, one has just to remove the task manager widget add the icon task widget: