Wow, there seems to be some huge misunderstanding. EVERYONE should be saying yes to this question.
For those of you say no, I'm respectfully telling you that you don't fully understand it. Here is why
AT&T: I sign up for 2 year contract get new shiny iPhone for $200. I pay $Z a month for 2 years, no questions asked. When 2 years is up, I'm told I should probably resign for a new shiny iPhone for another 2 years.
T-Mobile: I sign up for a plan for $X, NO contract. I tell them I want shiny new iPhone with that no contract plan, they say no prob. They tell me I pay $100 up front for iPhone and that on top of $X, I also be paying $Y for the phone (which you can pay off at anytime). So right now I see how much my plan is costing me, and how much my phone is. After 2 years, I've paid for the phone. Rinse repeat.
The Difference: With AT&T I don't know how much is going to the iPhone a month, therefore I don't know how much my plan ACTUALLY costs me. When 2 years are up, if I decide to NOT upgrade, I'm still charged the same amount even while I was supposedly paying off the phone w/ AT&T. WHY? Lets keep going. Not only are T-Mobiles plans cheaper with an attached phone payment, the iPhone itself is cheaper... $75 cheaper than you even get from the Apple Store. Did I mention, NO CONTRACT. Also, numerous calculations show you will at least be saving $600 over 2 years. Unless you wipe your butt with dollar bills, $600 is a bit of cash. Let's not get into "but they're only in 7 cities with LTE"... the other carriers use much different standards for what city coverage means with LTE. T-Mobile says ENTIRE city.
So with all this in mind, you have to take into consideration coverage. Places will have different coverage so you might not even have real options. But if you do the math, you'll always come out better with T-Mobile. Heck, even if I don't stay with T-Mobile in the future, I know I'll sure as heck go to their stores to buy an iPhone (which gets unlocked right there when purchased in full) since it'll be $75 cheaper than an Apple store.