In Ubuntu 11.04, installing the GNOME Shell desktop is not supported because it uses GNOME 3, while Unity still uses GNOME 2. As of 11.10, Unity uses GNOME 3, and GNOME Shell is now easy to install. You won’t need to download and install an excessive number of packages, everything is in the Ubuntu repositories, and GNOME Shell will coexist perfectly with Unity, even sharing the same theme.
Search for and install GNOME Shell from Ubuntu Software Center, or install the package gnome-shell from the terminal:
sudo apt-get install gnome-shell
Like with other desktop environments, you can start GNOME Shell by selecting GNOME from the gear menu before you enter your password to log in.
I’ve only played with GNOME Shell briefly on a live CD before this, but it seems to be working well on Ubuntu 11.10 in VirtualBox. Now that it’s available, I’ll have to try using it for a while once 11.10 is installed on my main system.