$99 Down....but what if I want a different phone?

TMobile has unleashed their new phones plans on us and it seems great. Put a little down on a phone, and pay a bit each month. Sounds great. Here are my questions I need answered:

What if I want a new phone later?

Do I return the first device?

Is it like a lease on a car?

Do I still owe the remaining balance should I decide I no longer want the phone?

These are just a few questions I have.

Honestly though....I don't see too many advantages to switching to TMobile. So what if I don't sign a contract? How many people switch carriers? Are two year contracts really that bad?

I get the idea of 'choice' for switching later should I not want to stay with my carrier, but who actually stops paying for phone coverage for a month and then needs it back on? Maybe someone in the military, or another very specific use case, right?

If that's the case, then the mass appeal isn't there.

TMobile has no real advantage over its other competitors. Their network is mostly urban based. Only today have they lit up LTE.

How is that better for the majority of people? What about the millions of people TMobile simply doesn't try to cover?

I looked at their coverage map and there's a huge swath of uncovered area stretching from Montanta diagonally to southeastward to Alabama. (Not to mention the fact that TMobile has tried to deceive people by coloring the entire map pinkish to give the appearance of coverage, what a joke)

So, those of you on TMobile or are considering the switch...why are you doing it? Also, could you answer my questions?