I had not found any really good impressions about KDE for Ubuntu (or Kubuntu) ever since I used 5.04. I have better impressions about KDE since running KDE 4.2.2 under Ubuntu 8.10. But the startup time and logout time are both a bit slow there.
Apparently Kubuntu 9.04 is better. It is noticeably faster than KDE 4.2 under Ubuntu 8.10 and Fedora 10 during login/logout and normal operation. So that is a big positive. KDE 4.2 is really quite sleek and beautiful compared to plain Gnome although the smoothness is still not as good as Gnome.
Initially I got no sound when playing MP3, similar to what I met when I installed Arch Linux the first time. The log-in/log-out sound are there. The mixer shows the correct device, just got no sound from other applications. By comparison, Gnome just works without any issues with 9.04, as always even since 5.04 (ok I admit I had some problems with 5.10 initially last time). I had to use Mixer of Gnome to "kick-start" the sound. I got sound but Dragon player still does not work. KDE told me this during after startup that"Notification from Phonon: The audio playback device HDA Nvidia (ALC1200 Analog) does not work. Falling back to default." I checked back the 8.10 installation and actually the problem with Dragon Player was there already.
Overall KDE4.2.2 under Kubuntu 9.04 is actually quite good even though there are occasional stability problem here and there. I will run KDE session more often than last time. Hopefully KDE 4.3 will be even better.