From reading the bug it sounds like KDE4 is getting close to being ready to hit the tree, which is awesome. Foolishly, I decided to try it early from the overlay last night. It was a total disaster. Things were crashing left and right, panels would resize themselves to be fullscreen (with hilarious results), half of my apps didn't work at all. I found three or four ways to bring down the entire X server. It took me many hours to get KDE3 running again. This is totally to be expected from installing masked packages as I did, so it's my own dumb fault, it was amusing and I wanted to get a taste.
I'm afraid it's going to be inevitably difficult or impossible to migrate cleanly from KDE3 to KDE4. I had the same problem in Kubuntu when I tried a while back. KDE is so huge and so many things link to it or interact with it that it's going to take a year to track down and remove all the cruft after the switch.
I couldn't even import my old KDE3 color schemes or Konsole color schemes into KDE4, which was surprising. QtCurve was un-configurable, dekorator didn't work, and so on. I didn't get far enough to figure out if my preferred icon themes work or not. I didn't realize they broke backwards compatibility to that large an extent, but I maybe it's to be expected.
There were other problems that were seemingly due to the lingering immaturity of KDE4. I can see all the pieces there which are going to allow people to do really neat stuff eventually. In the meantime KDE4 feels horrible.
KDE4 fonts look nice though.