1. MUST be activated with one of those four carriers. No way around it
2. Cannot be used if you have an outstanding contract on that line. If you try, it'll tell you how much you have left to pay off. There will be no way to pay this through Apple's system, you have to do it through a carrier. Also no way around this
3. This is a loan you are taking out, so it does hit your credit
4. Must use a credit card (yes CREDIT CARD, not debit) for the initial transaction. The first transaction is the first month's payment plus tax. After the first payment you can go into the bank's portal and change the payment method to a debit card or bank account if you want to
So if you are still in contract, you'll have to pay it off before you do this, you can't just put your sim in.
From what I gather from the details on the Apple website is that you'll essentially be taking out a personal loan from Citzens Bank for the cost of the iPhone with AppleCare+. I don't care what type of value Apple tries to add to it, it's not worth the inquiry on my credit profile nor the lowering of my credit score because I'd be obtaining additional credit. For some it may be worth it but for me, no thanks. The terms seem to be similar to the Verizon device payment program only VZW doesn't leave a hard inquiry on your credit report nor does it lower your credit score.