Yesterday evening I planned the upgrade of a server I share with a few friends. This server already hosts some bigger sites like Mechelenblogt and Hackerspace.be.
The server ran Ubuntu Jaunty but we needed to upgrade to Karmic because of new features in Apache we wanted.

Suprisingly Ubuntu has a better day to upgrade than the debian apt-get dist-upgrade + change your /etc/apt/sources.list to reflect the new repositories.
You simply need to type do-release-upgrade followed by enter and an interactive upgrade process will do all the work for you.

There were no issues at all with the upgrade. With all the reading and double checking what it did the upgrade took only 50 minutes. Ubuntu++, Linux++, opensource++ is my conclusion.