Yeah, it's pretty well-known that, outside of major metro areas, and perhaps along a few major rural thruways, T-Mobile coverage is marginal-to-non-existent. Then again: When my wife and I took a trip to northern Michigan last summer: Once we got north of about the Thumb region, and particularly when we got outside cities in the U.P.: Sprint was non-existent, too. (Expecting this is why I activated On Star in the truck for that trip.)
Tho it sucks, truth is this: If you depend upon coverage, and particularly for business, Verizon and "at&t" are the only rational choices.
We're pretty happy with TMO, but they could lose us. Because of the coverage issue, that happiness is kind of marginal-ish. (Which reminds me: They never got back to me about the dead zone in which our home resides, which, according to their own map, should be strong, and a new dead spot in on my daily commute route recently appeared.) If Sprint buys TMO, and particularly if it looks like the combined company is going to look more like Sprint than TMO: We'll likely be gone. If TMO doesn't improve their footprint, as they've been promising: We could be gone. If TMO doesn't give us better options for un-bloatware-infested devices: We could be gone.
We really like TMO, but a non-functional service isn't much use, no matter how much we like the company.