You can get $300 off iPhone 7 or 7Plus with trade in of a 5C, 5, 5S, 6, 6Plus, 6S or 6S Plus. You DO NOT NEED TO UPGRADE to T-Mobile One plan. I got $18 credit immediately for an iPhone 5C and will receive $282 (in 6-8 weeks) on a Mastercard, which can be applied towards balance of iPhone 7 or 7Plus. I think this is worth it if you have one of the iphone 5's or possibly 6 or 6Plus (say 16GB) if you do not want to hassle with selling. I do not think it is worth it with a 6S model personally. Again the good thing is you can trade in iPhone 5C, 5 or 5S and you do not have to change to T-Mobile One and you ARE NOT required to add a line. As far as I know it can only be done in T-Mobile store and you must start the 24 Month EIP plan.
UPDATE: Everyone make sure you process the rebate yourself. The store salespeople sometimes say they will process it for you but if you do not get an email confirmation in 24 hours then do it online yourself via the T-Mobile website.