Google Fi is a MVNO Cellular Service Provider. With supported devices, they use Sprint, T-Mobile, US Cellular, and Wifi capabilities to handle voice calls and SMS. Their service is $20 for unlimited voice/SMS + $15 per additional line. Data is $10/GB across all lines. Data only SIMs are free, and do not have a $15 fee, but counts towards your data usage. After 6GB, data charges are capped, and at 15GB, data will be throttled. This will mean a single line will pay no more than $80 + taxes.
So far from me using the service, calls and SMS come through reliably, and quickly. I made a few calls over Wifi, and handled calls over cellular. I also have good Sprint, and T-Mobile coverage in the city of Pittsburgh PA. It’s not so much of spectacular, but the service works as expected which is being used with my Pixel 3a XL. When on Wifi, the service connects to Google’s VPN which prevents ISPs and would be Wifi hackers from seeing what you are using your data for.
One bad thing about it, is you have to choose between Google Fi, and Google Voice. You can’t have the best of both worlds. Call forwarding will ring your Fi number, and all other phone numbers. In addition, there is no way to filter Anonymous Callers from calling. With Google Voice, you could decide which group rung which phone line, and can set Anonymous Callers to not ring any phone – therefore sending them to voicemail. You can’t forward calls from your Google Voice number to a Google Fi number without doing some forwarding trickery that only adds to the possible problems you could have if something isn’t working correctly. If I had best of both worlds, I would continue to send Anonymous Callers to voicemail, and very important callers (such as doctors) to ring both my Fi number, and home phone. This sadly is not the case.
With Google Fi, taxes are extra, and for Pittsburgh, one could expect 25% of the bill to be added as taxes. This would mean a $30 bill will become about $38. Not so much to gripe about, but considering companies such as Cricket and T-Mobile includes taxes with their pricing, this is something Google should consider looking into.
For full network compatibility, one has to purchase a select number of phones. Otherwise, if you use any GSM/LTE phone, it would only connect on the T-Mobile network, and calls may not carry over Wifi. This will mean if you have a poor T-Mobile connection, your call quality could dip. Compatible devices for sale at time of writing this page includes the Google Pixel phones, Moto G6 and G7. If you already have one of these phones, then you should have full service options.