This newly announced change, effective the next billing period, adds unlimited free data once you reach 6 GB of data usage ($60 worth of data + $20 talk and text). Essentially capping a single user's bill at $80. There is also the caveat that data will be throttled after reaching 15 GB.
As a low data user, this is not applicable to me, but it's good to see Google Fi actively working to improve their offerings. More info available below.