I'm thinking about switching from att to tmobile. Will I be at a big disadvantage without having a b12 device? I'm mainly on the east coast and parts of the south
It depends on where you are. If TMobile doesn't have coverage where you need coverage then, yes, it will be a BIG disadvantage. (So will TMobile's trick of connecting to a media server [like YouTube] at full speed, then shifting to a lower speed. You get lag and stutter. All the video you want - but not too useful.)
Riding my motorcycle up in the Adirondacks my battery went dead and I was pushing it down the road. A passing motorist told me that I wouldn't be able to call for help unless I had Verizon. I checked and there was no T-Mo or Sprint signal at all. I believe though I have had AT&T there in the past. In any case, don't believe the T-Mo hype unless you stay in major cities or along major highways--there are big holes! The further from major population centers you get the less likely you are to find T-Mo, in general.
So true, and I have found that RootMetrics and such don't work well out in the sticks either--not enough reports. It is mountainous around here and a slight bend in the road can just end your cell phone signal, even when on major highways. If you happen to live in the wrong valley you might have no service while down the road a bit you have great coverage because you are in the line of site to a cell tower. You can find the same thing in cities due to structures blocking your signal. The gold standard is to check it out for yourself using a prepaid phone in the locations you need service.One network may work great for someone whereas another would be awful. Just depends on needs and where you play/live/travel.