I was on Project Fi for little over 2 years. From price point it's the best deal. Never paid more than 50 bucks on avg (with 3 gigs of data). Each gig is 10 bucks even if you go over your plan. You get credits for unused data. Called back home to Nepal directly @ 0.09/minute which is as cheap as calling cards.
Call quality was good for few months but then it started going downhill. It became so worst that people on other side stopped hearing me or audio got jittery. I think the issue is with their network switching in an attempt to find strongest network in your location. As you know Fi works on wifi, Sprint, T-mobile, and US cellular networks. It switches to any of those networks depending on signal strength. Since your network constantly bounce between wifi, Sprint, TMo and US cellular voice quality is lost in between those network switch.
And remember cell tower are not reliable therefor your phones signal bar always fluctuate every minute? In between those fluctuations Fi tries to switch to strongest network. That drags the call quality in a worst possible way you can imagine.
Depending on how solid network coverage is from Sprint, Tmo and US Cellular in your area plus how fast your internet speed is, call quality varies.
The best customer service you'll ever encounter. I recently moved to one of these so called nations fastest provider and on the very first day I realized how badly I am going to miss Fi's customer service.