It's been only 3 days since the Gnome team announced the alpha of Gnome 3 extensions and there are already quite a lot of extensions which are making Gnome 3 more useful. I had love and hate relationship with Unity, I loved it for saving some space by using Mac like global menu, but at the same time it was counter-productive when you are using multiple windows. That was one of the reasons I switched to Gnome 3 as soon as Ubuntu 11.10 make it easier to use Gnome 3 (I am also using openSUSE 12.1 and Fedora 16). What I did miss was a little bit more space on Gnome 3. Not any more.
fpmurphy has created a Gnome 3 extension called Auto Hide Top Panel (uploaded to Gnome Extensions by erguille), which hides the top panel and give you full screen - it not only saves time but also removes distracting elements. When you take the mouse on the top, the panel will appear. Since all the menus remain with the respective windows (unlike Unity's global menu) there is no productivity loss.
I just enabled the extension and find Gnome 3 Shell even more useful than Ubuntu's Unity. I think Ubuntu team should make it a priority to offer a set of tools to customise unity -- especially to disable the global menu in non-maximized windows.
Extension at play:
Once installed, you can disable/enable the auto-hide -- just double click on the top panel.
Kudos to the Gnome 3 developers for paying heed to what users want and creating the much needed extensions.