I have had T-mobile for almost a decade and never had any problem with them. They are a GSM network so you can just swap out your sim card if you want a new phone, without having to jump through any hoops. They even went so far to correct problems that users were having with jailbroken iPhone on their network when they were still relatively new, which is pretty incredible.

I have never had any problem with coverage in major cities, if you get out of populated areas you will see that you are typically bouncing off partner towers, so you may lose data service, but will almost certainly have cell service. Basically as long as you live in a large city you will be fine. I never have had any problems (at least noticeable enough for me to remember) with dropped calls, or flaky service where I live.

I assume other peoples experiences may vary, but I think they are hands down the best cell provider.