Samsung S7 Edge users (and probably other Samsung models too) might enjoy this small workaround to activate "OK Google" with a long press of your home button, very much like how Siri is activated on the iPhone.
[Previously, my long press was opening Google Now cards—not useful].
Step 1: Make sure both of these apps are downloaded. (Available for free at the Play Store).
Step 2: Setup your phone to trigger Home Launcher
Go to Settings -> Applications -> Default Applications -> Device assistance app and select "Home Launcher"
Step 3: Set Home Launcher Up
Open "Home Launcher" and add "Voice Search" [And remove whatever app might be already setup]
Go to "Home Launcher" settings and check "Auto start mode if only one single app selected" and click "Save"