Worst Kubuntu upgrade ever
I have just upgraded my Kubuntu installation from Breezy (version 5.10) to Dapper (version 6.06) following the instruction found on the release announcement web page. Results are as follows: the kubuntu-desktop package has been uninstalled (yes, it is a known bug but, really, lowering the upgrading process quality, aren't you?); my Eclipse installation has disappeared, at least from the KDE menu; sound does not work anymore; and what's worse, I get no more Internet connection at operating system startup. (I'm writing this from Windows XP, then.)
Don't really know how the upgrade could have been worse. My future use of Linux will depend on the chance of finding the time to solve (or search the Internet for someone else's solution) the problems the Dapper upgrade has caused. To people who find themselves less and less impressed with proprietary software: how's that for being impressed by the wonders of free software?
Update: more happy upgrades!

Clean the situation by doing a backup of your /home dir and nuking the root partition, then install the new version. As a matter of fact, you should have two partitions for / and /home, so it should be trivial to reinstall.
Unfortunately, despite the good job done by the Kubuntu people, KDE is still a second-class citizen in the Ubuntu world.
btw, remember when I told you that apt-update is more reliable than dist-upgrade? I wasn’t lying
Truth be told, I was indeed impressed by how clean and crispy the new KDE seemed to be. And I thought about doing a reinstall from a CD, too. But I was really too much upset by how I have been screwed (reinstalling the distribution would mean to also reinstall the ADSL modem support) that I decided I wouldn’t mess with Linux again for a few days, even if just that morning Konqueror (the only application that will be really missed, along with Kopete) saved my life once more.
Regarding apt-update versus dist-upgrade, I used Adept as the instructions told, so I don’t really know how the upgrade was managed on the lower level.
Update on the (Gnome) side of Ubuntu went very smoothly. Having to upgrade computers in different scenarios I experienced the whole range of upgrade options: update-manager, vi/update/dist-upgrade, dvd supported. The only things I got screwed were some Shorewall configuration files on a server: this dues to a syntax change in the Shorewall major upgrade from 2.something to 3.0, and a lacking packet replacing for the synaptics touchpad driver.
I also uninstalled my custom built ubuntu jdk package and now I am succesfully using the ubuntu provided jdk package (Eclipse,Netbeans and everything else working fine). The same for VMWare Player.
By the way the installation I am running on this laptop dates back to pre-warty times.