My experience is anecdotal, of course, but we've been using Google Fi for years. It uses the networks of T-Mobile, Sprint (now owned by T-Mobile), and U.S. Cellular, automatically switching to whichever has the strongest signal at the time—and it does so seamlessly. It also switches to Wi-Fi calling when that's the better option.
Since T-Mobile acquired Sprint, and U.S. Cellular's coverage is mostly in the Midwest, the vast majority of our cellular service now runs on T-Mobile's network.
We've traveled extensively—to Canada, Mexico, the Caribbean, South America, Europe, and all along the U.S. Eastern Seaboard from Maine to Florida, as well as out West—and I've never had any real complaints about coverage or performance. In fact, my wife sometimes complains about spotty service on her work-issued Verizon phone while our personal phones are fine.
Whenever we're in an area with truly no coverage, both phones lose signal, which makes sense. But there have been plenty of times when her Verizon phone shows no bars while our personal Google Fi phones have strong signal. That happens more often than never.
