Just want to add that I've been using KDE 4.2b2 since the packages popped up on the kubuntu-experimental ppa and it's been a really smooth ride. I started off with a clean slate so no older KDE 4 settings or packages, and the only instability I've faced so far is with Dolphin that crashes every once in a blue moon.
Also, there's the thing with the video garbage that's yet to be resolved (since popping up in the first 4.0 betas). There is a workaround, but performance drops. See https://bugs.kde.org/show_bug.cgi?id=170462
and https://bugs.launchpad.net/ubuntu/+sour ... bug/254468
. At least it's getting attention now.