This will be Kubuntu’s first release with the new Kmail2 that uses Akonadi to store all your mail related data (and more) in a database. It is slow, unreliable, and extremely resource hungry. It can be made to work, but the situation is not good. My advice is to be careful about upgrading and test things in a VM/on a spare computer before upgrading to 11.10 if Kmail is important to you.
I put a lot of effort into making sure we had every upstream bugfix available for Akonadi and the KDEPIM related packages, so I think this is as good as it gets for now. Hopefully it’ll be better in KDE SC 4.7.3 and 4.8. In the meantime, please make sure to give feedback to bugs.kde.org.
Personally, I’m caught because hardware support for my laptop is much better on 11.10. I’m thinking I’ll probably go with a 10.04 vm just for kmail and do my best to ignore kmail2 for now. Good luck.
Update: Improved the title.