The Google Now launcher has been exclusive to the Nexus 5 since Android 4.4 Kitkat was released. If you own a Nexus or Play Edition handset that runs KitKat you’ll be able to get the launcher in all its glory. Everyone who has the Nexus 5 has loved the new launcher which allows you to say “Ok Google” from any home screen to automatically launch Now’s voice command functions.
If you’re running Kitkat already you already have most of the clean features of Android, and the app download only gets you the fast access to Google Now. Now the next step will be getting the launcher on more phones, even if it still requires you to have KitKat. So if you feel the need to be able to use Google Now while on the homescreen do install it. I hope Google will expand the Ok Google to anytime the phone is on.