Just wanted to post my experience with GoogleFi.
We got it about 6 weeks prior to a trip to Ireland so we could try it out to see if my wife would want to switch to it indefinitely.
We used it on an unlocked iphone7. Setup was easy. I believe voicemail on the phone was the only feature it did not support.
It had decent cell reception at home, but data was always slow on T-Mobile 4G LTE. As compared to a similar iPhone on VerizonLTE. We compared it around North and central Alabama. Rural and metro areas.
In Ireland it was worse. After talking to locals and showing them the phone, it made more sense.
There are several carriers there, as with most European countries. Some are better than others. I believe 3 was their premier brand with best data network. When data was off it would connect to a premium carrier with LTE. Once you turned on data, it switched to a different carrier with only 3g service. And 3g was slow. It was so bad we ended up paying the $10/day to just use our other Verizon phone.