Recs.
Updated
Specs
Pros
Pro Easy theming
Changing the look (and feel) of Gnome Shell is easy, shell theme, icon, windows and graphical elements (gtk). individually for each user. Mostly its installing some packages or unpacking some archive to a themes folder and using selecting the new theme in e.g. gnome-tweak-tool. There a lot of really good theme on deviantart.
Pro Customizable with easy-to-install extensions
With the right plugin enabled in your browser (comes with Firefox by default) you can browse and install with two clicks the many Shell Extensions available from http://extensions.gnome.org. These are listed automatically based upon the version of Gnome Shell you currently have installed.
Cons
Con No taskbar by default
GNOME doesn't have a taskbar by default, and might be confusing to people migrating from Windows for that reason. It is however possible to install it through http://extensions.gnome.org.