Just one problem if you have a situation like mine.
I have a Google Voice account that I use for everything. If my cellphone number changes (and it has about 4 times in recent years - a move a change of carrier, etc.) I still have the same number - my GV number. It forwards to both my cellphone and my landline.
With Fi, the GV number is the cellphone number. And you can't forward from one GV number to another. So I have to have the GV number transferred to another account, port the cellphone number from T-Mobile to my new GV account and use hangouts on the phone to get calls on the original number (that's already forwarded to my landline).
Google isn't a big-time-player in the telephone business yet. (But it's saving me $30/month most months, so I can't complain - except for waiting for the port-in.)