Well they've got the least amount of coverage in the US but they have great customer service. So the question is do want your phone to work where you need it? Or do you want great customer service telling you your phone won't work there, here, anywhere?
They do however have some of the cheapest plans though.

what carrier do you have and what phone do you use?

Don't listen to him. Tmobile has great coverage and are just as good as any other carrier. But like with ALL carriers it doesn't work for everyone, so go with the carrier that works best in your area. If you want you can go into a Tmobile store and they can do a personal coverage check for you. If it looks good then I would go with Tmobile. We have the cheapest plans around.

T-Mobile is primarily for people who stay & travel in major cities. For those who travel through most of rural america will find themselves with either no service, or doing a lot of roaming. T-Mobile show little interest in covering rural markets. So for those who travel through most of rural america should look at someone else. Besides, T-Mobile is joining at&t at terminating users who roam too much.

It's best for people to look at where all they travel to decide if T-Mobile is right for them, based on T-Mobile's native coverage map.

dont listen to him. ANYONE who says that t-mobile coverage is on par with sprint, att or vzw is full of it. and everyone knows is but no one wants to admit it. you get what you pay for. metro pcs is like $50/month unlimited everything, but you cant use it nearly anywhere. maybe for the money t-mobile is a good value, and like dave73 said... if you live in a major city, and dont leave the major city, then t-mobile should be ok. drive from city to city, and you sol (sh*t outta luck)

but they do have pretty cheap plans

it also really depends on what your looking for too. if you want data and txt and talk alot your better off going with sprint they have cheaper data/txt bundles or even data/txt/voice bundles but if your into all of that then t-mobile will be right for you. sam as if your not good handling your bills. tmobile will cut your service so you be getting extra large bills while sprint will keep your service on untill you go over your spending limit. if your the type to drop/lose/orphysical abuse your phone tmobile is not for you. insurance deductibles are too high. as for sprint its only $50 down no matter the cost of your phone. im only comparing those too because i know them better then anything. i had verizon for a short while but they had to...
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tmobile is cool but it always seems like their is alway on outage. if you go to mytmobile.com its shows you all the outages ive typed in ramdom address and it seems like their is alway on outage. cust from west coast will have a problem with service and have to be told its a network issue yet the issue clearly states new england area, but it affects all. 😁 😳 🤨

thank you for all of your info. Ive had sprint before and wont go back to them. personal reasons.