It cost me a lot of effort and several misconfigurations, but it was worth it.
First of all, I needed to change my debian repository from Lenny to Squeeze. This is where problems started. I didn't like having all those new packages to upgrade, so I decided to upgrade most of them. The problem was that one of those packages overwrote my libglib, and suddenly I couldn't get any GTK applications to run. Like, you know... synaptic - the package manager.
I managed to reinstall the previous glib by switching repository back to Lenny, and doing some package juggling in aptitude (the commandline package manager).
Okay, we're safe again, I can boot up firefox.
But oh, no, another problem. The Squeeze version of Synaptic has a segfault problem on certain packages, so I had to update with the SID (experimental) version, where the bug had been fixed. This one gave me a lot of headaches.
After I managed to get it fixed, I updated most of all the applications (except those with CVS / SVN numbers in their versions).
So far, so good.
Finally, there was KDE. First I downloaded all the pertinent packages in synaptic by choosing "download packages only".
I backed up my ~/.kde3 directory...
Most of my settings were saved - except the keyboard. And some autostart files that I had previously disabled also ran, so I had to remove them by hand (I forgot which file I edited).
Next, I realized that the settings weren't so easily found. I still can't find where the "start feedback bouncy icon stuff" is located.
I had to reboot because a game messed up my display (hint: Don't try to run KQ, it forces the full screen on you, and if you kill it, the screen keeps being messed up).
I also noticed that my numpad doesn't work so well in konsole. Why is that? I don't know. I guess I'll find out later.
Another setting that I had to reset was the shortcuts like "execute", the task manager and the "start menu" (the big "K" button).
After setting those correctly, I also realized that some settings in the System / System Settings are set to read-only, there's no way to run the program in root mode, I don't know how.
Another downside is that the quicklauncher widget is quite hard to set up, you can't just drag and drop like you did in KDE 3.5. You need to navigate through the right click menu.
The new Plasmoids are pretty cool. Unfortunately, there are too few of them. I guess KDE 4 is still seen with bad eyes by devs.
Another quirk (in my last reboot) is that you need to click twice on the quickstarter to actually see your apps. I hope this happens only in THIS boot.
Other than those quirks, I find the overall look-n-feel to be quite pleasing to the eye.
I think the upgrade was worth it. I was getting bored with my old kde 3.