Very interesting read, particularly in where the service providers are expanding their network. It speaks a lot to the each company's priorities and confirms a bit of my personal thoughts on each of the companies.
Verizon - I had Verizon service from 1996 through 2008 and the one thing you couldn't really complain about where I lived was reception. Billing issues and customer hostile behavior were the reason I ditched them [0] but there wasn't a moment I can recall grabbing my phone and not having a solid signal everywhere I traveled in the US. They were laser focused on providing a modern, reliable network. Quickly moving to increase capacity - they've always had a pretty solid focus on providing a solid network.
AT&T - I have a personal hatred for this company after numerous problems with services other than mobile. Their attitude was best summed up by a Saturday Night Live skit[1], "We're the phone company. We don't care." I've never used their services if I had a choice (that was probably the single biggest reason I didn't purchase an iPhone, originally) because every time I've been forced to use one of their offerings, a variety of service related issues (not helped by their customer service department) has made them unavailable for long periods of time. It's disappointing to me that they seemed to think it would be a good idea to halt wireless expansion and redirect that money to the DirecTV acquisition. It seems that couldn't have come at a worse time considering the rapid expansion of wireless.
T-Mobile - This was a little surprising and appears to have not been intentional. They had no choice but to setup a large number of cell sites due to the limits of their spectrum. I was a T-Mobile customer (and, technically, still am one now that I'm on Fi) and I've never had real problems with their service outside of 4G taking a while to get coverage in all but the most densely populated areas.
Sprint - As a Fi user, that's as close as I've gotten to Sprint service. They had an interesting network in Michigan. The coverage is spotty in areas that most other networks are well covered and good in areas that aren't covered well by anyone (there's an area of the thumb off of M-25 that is dead to AT&T, T-Mobile and Verizon but works on my Fi phone). Legacy equipment is all over their network, likely because replacing it is beyond their financial means at the moment. I don't see it getting any better. They're a fourth player in a market with barely enough room for three. In a healthy market, you'd expect a player with poor service to be substantially cheaper but last I checked (and I'll admit it's been a year or two), there wasn't much of a price incentive for accepting the lower service quality.
[0] Technically, I was an AirTouch customer in 1996 until whenever Verizon purchased them. Back in the day, they had the best network in the area I lived in. My bill was around $150 and this put me on the high end for their already expensive service. At some point they put people above a certain service plan into an arrangement where dialing 611 resulted in being connected, within five rings, to a human being. The first time I did this after they switched it around, I stammered a bit due to not getting a seven-layer phone tree. Dealing with their perfect English speaking CSR staff was a delightful experience and it was gone shortly after Verizon took over.
Bear in mind that Sprint is one of the options. Fi uses both T-Mobile and Sprint and intelligently selects between the two based on network reception and performance (the latter being the coolest feature -- my 4G speeds are consistently fast).
Sprint was probably cheaper than Verizon and AT&T, making them a more economical choice.
The second reason I have no solid data on and have been meaning to investigate further since the anecdotal experiences I have had with Fi support it (but have no idea how to get good data on this[0]). I think T-Mobile's and Sprint's networks are complimentary and Sprint has reach in places where nobody else provides good coverage. The concrete example of this is my parents' home in the thumb area of Michigan. Every other provider has a dead zone near Fort Gratiot going for about 2 miles on M-25...except Sprint. Every provider has service in the city the home is located in, but only AT&T and Sprint cover it well (4G in most parts of the city except for low lying areas where only Sprint and AT&T seem to get any service). My travels through the thumb with T-Mobile resulted in several areas where coverage dries up but the same travel with my Fi phone yields good service in most places[1].
[0] The provider maps are notoriously terrible, indicating theoretical service where it isn't from a real-world standpoint. There are crowd-sourced apps like OpenSignal that measure actual coverage, as well, but they had no good data for this area because it requires one to run the app, which is already a fringe "geek" thing, and the app has to be polling at the time you enter the area. I'd spent almost every day of an entire summer up there with my phone and there was no data on signal quality for T-Mobile. This is beyond the fact that some areas reported data that was plain wrong (likely outdated or resulted from a phone model having a different/significantly better radio than mine).
[1] I checked the same month I switched since I had been used to having no data for about half of the drive, causing Pandora to quit. To my surprise it worked for all but a couple of miles and in those spots, I still had 3G. We also have friends up there who run AT&T (which gets 4G almost everywhere except our property) and my family is on Verizon (which has reasonably good service when it's confused about being a Canadian -- a problem Fi has as well).
