I switched from Verizon to T-Mobile a couple years ago, and where I live (Oregon), T-Mobile's service is absolute garbage compared to Verizon's. It was fine in the dense parts of the city, but anything outside of that was a total gamble. Driving to the mountain, the beach, or really anywhere outside the metro area nearly guaranteed a lack of service all or most of the way. I even had poor reception in parts of my house, and would sometimes lose calls in my kitchen!
I recently switched to AT&T and am much happier, but there are still occasions where my wife (on Verizon) has service but I don't. I've also heard similar anecdotes from people I've met in other parts of the country, so I don't think Oregon is the only state for which this is true.
(Before you say that this was a problem with my phone, I changed phones once while I was still on T-Mobile, and the service did not improve.)