Earlier quoted context omitted.
The only thing with Fi is that it is T-Mobile, and you'll always be in a lower priority block of customers compared to people paying for T-Mobile directly, which mean you'll see slower traffic in congested areas at peak times (including e.g. during rush hour traffic).
If you're travelling, I find it hard to beat Fi. You literally land in a new country, turn your phone back on and you get a "Welcome to [country] - your data rate is the same" message almost anywhere. Personally - I've flown from Taiwan to Brazil to Amsterdam and then back to the US and I don't have to think about my phone. It just works. --- Outside of the travel use-case, I would also probably pick something else,…
I still have my Sprint plan. This is how it works by default. (Sprint + gvoice = google fi; before gfi you could merge your gvoice and sprint accounts which was really cool. Then they cancelled that and started gfi) Since the TMo merger, I suspect gfi is still using the Sprint stuff.