I used Project Fi in addition to my T-Mobile (unlimited data + international roam) phone for a recent RTW trip.
In the "first world", Fi was more expensive (I am a heavy data user -- Fi s $10/gig) than T-Mobile, as they both roamed in the same places. This is US, Canada, Europe. The comparison suffered because T-Mobile was doing a "4G international speed" promo at the time, so T was the clear winner. Fi allows unlimited tethering, Tmobile caps high-speed tethering. In countries without the 4G promo, the speed was usually an unusable 2G, which couldn't even keep up with Google Maps.
In West Africa, the T-Mobile phone was often crippled by speed (2G ish) and often times the Fi phone could do full 4G. That was when it worked at all (Morocco, Ghana, a few others) -- when one had no service, the other had no service either. West Africa is dominated by the french carrier Orange, with South Africa's MTN close behind. From Senegal to Nigeria, it was usually much cheaper to just get an Orange Sim and a data plan -- usually 3G speed and a few bucks a gig.
In Southern Africa (ZA, Namibia, Zambia, Botswana), the Fi data price was cheaper than any local sim (except Zambia IIRC) so it brought a convenience benefit. It also whipped T-Mobile's speed being 4G vs 2G again.
In Asia, Aus, and NZ, it was the same -- Fi was fast and expensive data, Tmobile was slow and "included" data.
There was never a place where Fi had coverage and Tmobile did not.
There was often a case where Fi had very fast data and Tmobile had slow, unusable data. T-mobile offers high-speed data passes for an upcharge.
The Fi phone we took (Nexus 6P) was a battery-devouring piece of garbage that I sold immediately on our return.
So in summary: Fi is good for people who need fast data and don't wish to pay Tmobile for the data passes or buy and use sims in other languages. Otherwise, $10/gb is really steep in most of the world, save the southern parts of Africa, where it is on par with local offering and "handy"
If I had it to do over, I would get a dual-sim android (like my current Moto Z Play) and still bring Fi as a cold backup, but I would expect to not use it. Fi doesn't make sense to me outside of the travel context.
$0.02