I took the plunge earlier this week and started the upgrade of my Ubuntu machines from Jaunty to Karmic. I have five machines running Ubuntu to process: two laptops, a modern twin quad-core workstation, and old Pentium 4 workstation, and my (really very old) server (Pentium 3, but new SATA controller and discs). I haven't started the quad-core workstation yet, I am trying to decide whether to upgrade or install from scratch.
The two laptops were the first to change, the upgrades seemed to go reasonably on both. The Lenovo ThinkPad T500 seems to have upgraded fine. The problem of the machine failing to start Compiz on login remains. This is really infuriating and the Ubuntu people seem to not care. The Sony PCG SRC51P/B upgrade has been disastrous, Bluetooth and WiFi now fail to work ever. Overall not good. Also the sound control/mixer that was standard on Jaunty and entirely fine has been replaced in Karmic by something completely inadequate. gnome-alsamixer is also severely inadequate. So if you want to control your sound system then it looks like "use another distribution, not Ubuntu" is the best choice.
The disc structure on the Pentium 4 workstation was not up to an upgrade - didn't have 3.8GB of free disc on the root partition. So I had to do a fresh install so I could repartition the disc. The install went fine. Compiz still fails to start on login and the sound control/mixer is totally inadequate, but I already mentioned those issues early. Did I mention how infuriating this problem is and how even more infuriating it is that the Ubuntu support does nothing about it. On the up side Emacs 23 is way, way better than Emacs 22.
Upgrading the server seemed to go fine. There is no longer a distinction between workstation Linux kernel and server Linux kernel but then who cares as long as it works. The real problem was that /etc/resolv.conf failed to get set up which caused many a bizarre problem. Once found though, it was easily cured and the server seems to be operating fine.
Now the real gripe: Launchpad appears to refuse to accept any bug reports against Ubuntu. I can post new bug reports using Launchpad for all the other projects and packages I want to but not for Ubuntu. Hopefully this is not a hidden censoring of anti-Karmic facts.