I have Fi, and I cannot WAIT to get the hell off of it! It basically switches between Tmo and Sprint networks. The switching is slow and glitchy, and your phone goes out of service between switches. When on sprint, you can only do data or voice, not both at the same time. It's infuriating as when you're on Tmo you can, and unless you have an app telling you what network you're on, you're always guessing. Oh, and I drop calls all the time, like literally, ALL the time. To the point where I tell people to text me instead.
YMMV - I'm in the metro detroit area (Troy), on a Nexus 6P.
Can't wait to get off of it. But...it's cheap.