Ubuntu 9.10 Karmic Koala is still in its early Alpha stage, but I bit the bullet over the weekend and did a clean install on my Lenovo 3000 N200 laptop. Although I've been running with a Karmic kernel on my laptop for several weeks and also upgraded my servers to Karmic very early on, it's only now that I decided it was time to go the whole hog and upgrade my laptop.
Rather than just do a rolling upgrade from Jaunty to Karmic, I backed up my home directories and some configs in /etc and then did a clean install from Alpha 2 and pulled in all the updates. Then I restored /home and did some minor tweaks to my configs.
Starting afresh is quite cathartic; I got rid of a load of old applications that I'd installed a while ago and don't use any more, and I also started afresh with ext4. Ext4 brings some more speed, especially when fsck'ing the drive on boot. Ext4 is noticeably faster when removing hundreds of files, for example rm -rf on kernel source trees.
Karmic also now uses the Grub2 boot loader; this is working well across a wide variety of machines, as can be seen from the Grub2 testing page.
With Karmic we also get Kernel Mode Setting (KMS) too. So far, this is working fine - you soon notice that there are less screen mode setting flickers on boot and suspend/resume is slicker. Audio works OK with the default audio player Rhythmbox and with proprietary software such as Skype - I've not yet noticed any audio dropping, so this is a good sign so far.
There's going to be a lot more changes made over the next months, hopefully we won't see many regressions on the kernel, but with all the change, stuff does occasionally get broken. So I encourage you to help us by testing the Alpha and Beta releases of Karmic so we can squish those bugs and get changes upstream good and early!