In the beginning this service was only available for Nexus 6 users.  Now google has launched project Fi in the USA. Google Fi is networks of the networks.  Its cost is $20 monthly for the basic plan with which allows users to Talk, text and use wifi.

If you pay $10 more then you can get 1 GB data.  If you are unable to use that data due to any reason then your money does not get wasted. Google will transfer $10 back in your account. Google is working with T-Mobile and Sprint to increase its coverage. Google is offering 4G services in big cities and 3G in smaller areas.

Only those people can use this service who get invitation from others.  If you live in the US then go and click on this link  https://fi.google.com/signup?u=0 to subscribe.