I just upgraded my Kubuntu 9.10 "Karmic Koala" from KDE 4.3.4 to 4.4.0 using the backports repository. A note of warning: take a backup.
It took me something like 4 or 5 runs of
apt-get dist-upgrade /
apt-get -f install to get back a working setup. APT complained about a couple of things (python-qt3-doc conflicting with some newer package which includes the same examples, and Bilbo having been renamed Blogilo) but eventually pulled it through.
I was smart enough to backup the .kde home directory before launching a 4.4 session, and I'm glad I was. First run: Plasma crashed hard on a segfault, and didn't restart. Ok, this happened with some past upgrade already, let's just move away the old .kde home... got back a default desktop. Playing around with widgets, I hit another crash. And another. The culprit was always the Plasma Desktop, actual applications were running fine. Eventually, I moved back the old .kde home except the plasma* files under .kde/share/config/. This gave me back all my old KDE settings, except for Plasma. Win, I thought, and merrily set out to recreate my previous desktop arrangement.
So uhm. I'll try to track down what exactly Plasma doesn't like in my setup (which was, for the record, with two panels on left and right edge; Lancelot and a couple of QuickAccess widgets on the left, task manager and shutdown button on the right; two Plasma "views" and 4 virtual desktops), but if you are planning a similar upgrade, make sure you put aside some time for it, and get a good backup beforehand.
(I guess the question now would be "is it worth it?". Well, KDE does look more polished, applications are faster to launch and more responsive... it does look like we're finally at the point where one could upgrade from a 3.5.x release to 4.x and feel a bit awestruck.)