Following Ana's howto I successfully upgraded to KDE 4.0.80 and have used it for 7 days now.
I was upgrading from Debian unstable. There were some conflicts preventing me from upgrading. I could only go ahead after some trial-and-errors. That is, trying to resolve the conflicts by manually adding package names to apt-get.
Overall, KDE4 is prettier than its predecessors. The best thing I find with KDE4 is the new package okular for viewing PDF and other format files. The review facility enables me to add highlight to some texts and comments. And those reviews can be copied to share with others or to be used on my other computers.
I've come across a few application crashes, and that's understandable as beta software. I'd say KDE 4 is quite usable and stable enough to me.
If you are interested in Okular, you can also just install Okular. That will remove package kdebase-bin-kde3. I'm not sure whether that will pose problems to me or not. So far I haven't rebooted or restarted KDE and everything seems fine.