Can I use an unlocked AT&T iPhone with T-Mobile? I'm finally fed up with AT&amp;T's exorbitant cost and ready to switch to T-Mobile.

I'm well past my contract obligation and have maintained my iPhone 5 in near pristine condition. 

Can I have this phone unlocked and use it with another provider?
AT&amp;T and T-Mobile both use GSM networks. 

So yes, you will be able to use your iPhone with T-Mobile.