T-Mobile is so good where I live and play that I don't even turn on the roaming option on my service.
The whole premise of this thread is roaming/what will happen in situations where one (specifically, the OP) might have to roam. The signal where you work and play is irrelevant, because 1) roaming is being discussed, and 2) hopefully nobody would be dumb enough to sign up for service through TMO if their work/residence/most frequented places are in the sizeable TMO coverage holes.
Then again that is true about AT&T, Verizon and Sprint that all have holes in their coverage. You would need to get a satellite phone if you expect to have coverage every. Also, in some high traffic areas even the over priced services like AT&T and Verizon have congestion and poor signals but they just don't care about fixing their services either with that logic....
Yes. Everyone has coverage holes, but who's likely to have more of them: AT&T, Verizon, or TMO? And especially compared with Verizon, it's not even close.
Congestion is a real issue too (albeit not the scope of this thread) and right now, you're more likely to come across congestion on AT&T and Verizon. But that does not mean it's not starting to happen on TMO. In fact, TMO's whole strategy of deploying its network almost exclusively in/near urban centers lends itself to congestion, unless they keep supplementing their bandwidth and/or employ AT&T/Verizon-like policies of limiting data plans, etc. So worst case, you end up with congestion on all carriers, but TMO with still the worst nationwide native coverage and laughable roaming limits.