In my experience, installing gnome-shell (or ubuntu-gnome-desktop) over a working Ubuntu installation can be dicey. For example, If you install both side-by-sid

In my experience, installing gnome-shell (or ubuntu-gnome-desktop) over a working Ubuntu installation can be dicey. For example, If you install both side-by-side the Unity theme can get a bit screwy, and the desktop-background just renders as a black rectangle.