I upgraded some of my Kubuntu systems to the newest version (Oneric Ocelot) and for the most part it went ok.
The biggest issue was the conversion of Kmail to Kmail2 and its use of Akonadi for its data store. The first time I started Kontact after the upgrade it asked if I wanted to migrate my data, I said yes and it promptly failed. This also happened on another system where I said no.
Now Kmail/Kontact won't start. Fortunately my mail setup is all IMAP so no mail is stored locally, and I've never been good at tracking my contacts.