I've been on Fi for just over 3 years now and I like it. I don't get good cell coverage at my house, so the WiFi feature is essential, and since I have Comcast I get access to a lot more hotspots. I also travel to Japan frequently, and the international coverage has bee great. I even got coverage on a small island called Ishigakijima, which is about halfway between Okinawa and Taiwan.
We have three family phones in a plan. We get off the plane in England and everythign just works. No buying sims. No progamming in new numbers. Calling back to the US (or anywhere) is dirt cheap. We get home and dont notice any change to our monthly bill.
We've been on Fi for almost two years now and I'm happy with it. There have been oddities, like difficulty connecting to carriers in Los Angeles, but otherwise it's been a plus. We spent two weeks in the UK and coverage was awesome.
I dropped Project Fi after two years. The coverage was poor in crucial spots in my area and the phone would only switch between networks when there was no voice signal, meaning you could have zero data and it would remain on the current network. When visiting Paris this summer there was only 3G available over the city. Wasting time resetting network settings produced no better service and I suspect that Google's provider in Europe (O2) didn't want to buy 4G in France. I had two Nexus 6P during the period as the first failed during a trip to England leaving without any coverage until I returned to the USA. Yes, you only pay for what you use, but for me I couldn't get adequate use out of it. Now I pay less for better coverage from another MVNO.
I've been on Fi for years and travel the world promoting it - until now. Like you flag in your review: customer service turned horrible when they scaled. I'm out $1,600 of credit and I can reach anyone that can do something about it. It's bewildering they would treat longtime customers like this.