It's finally happening. After months of leaks and speculation, the Google Now Launcher (previously known informally at the Google Experience Launcher) has been dropped into the Play Store. This is essentially just an updated stub app that will enable the home screen functionality in the Google search app on most Android devices. However, this isn't a universal release.
The first official version of the Google Now Launcher is only installable on the Nexus and Google Play Edition devices running KitKat.