I have been using this for a couple of days now and it is pretty good! I also have the Amazon Echo and although it has its differences, OK GOOGLE is wherever you are, and Echo is not as you can only access it in the place you put the bulky device.
Great to use when driving too as you can simply make calls, text, ask for directions without touching your phone!
Your Android device needs to have:
Google app version 3.5 or higher
Android 4.4 or higher
Voice & Audio Activity setting turned on