A quick update for Ubuntu 13.10 users: the latest Cinnamon from the stable PPA no longer breaks Unity so it's safe to install it in Ubuntu 13.10 (Saucy Salamander).
If you want to give Cinnamon 2.0 a try without breaking the Unity session, in case you don't want to continue using Cinnamon, you can now safely install the latest Cinnamon from the official stable PPA.
Previously, installing Cinnamon 2.0+ in Ubuntu 13.10 would completely break Unity: when trying to log in to Unity, LightDM would fail to load the session and the display manager would restart.
Previously, installing Cinnamon 2.0+ in Ubuntu 13.10 would completely break Unity: when trying to log in to Unity, LightDM would fail to load the session and the display manager would restart.
To install the latest Cinnamon (2.0.12 at the time I'm writing this article) in Ubuntu, use the following commands:
sudo add-apt-repository ppa:gwendal-lebihan-dev/cinnamon-stable
sudo apt-get update
sudo apt-get install cinnamon
More about Cinnamon 2.0 in our previous article: Cinnamon 2.0 Released, Becomes An `Entire Desktop Environment`.