I would probably pass too. I was looking at the terms of the new offer:
It looks like they've updated the terms:
https://promo.sprint.com/Registration/ContractBuyoutLanding?ECID=vanity:joinsprint
Bring your number to Sprint and activate on Easy Pay℠, iPhone for Life Plan or Sprint Lease.
Turn in your last phone in good working condition within 30 days via the return kit you will automatically receive once your registration for this offer has been approved.
Note: if you’ve already turned in your phone(s) via Cut Your Bill in Half Program a return kit will not be sent to you.
The phone you turn in must be the last phone you used with your prior carrier and be the phone tied to the bill that you are uploading for this Reward Card.
If you do not turn in your last competitor phone, you will be charged back up to the amount of the Reward Card you received.
They clearly state that it needs to be the last phone you used with the carrier and the phone tied to the bill you are being reimbursed for.
However, this would seem to be added to prevent those on phone payment plans from not turning in the phone they are making payments on. The previous carrier's bill would also normally state the name of the device. It is possible that those reimbursement costs could be $700+ depending on how new the phone is.
However, it is still unclear how they will judge those on standard two year contracts with ETFs of $350 or less. If they want the last phone you used with your carrier, it could be anything and it's possible that someone could be stuck in a new contract with a high ETF, but their current phone is low value because the original 'high value' phone may have been lost, stolen, or damaged.
Of course, someone that is financing a phone may have also been the victim of theft or damage, but if your bill says that you are financing a Galaxy S6 and you send them a GS3, it would be easier for them to deny or charge you later for the amount of the rebate card.
Oddly enough, here is another current Sprint ETF Buyout promo that makes no mention that turning in your previous phone at all to be eligible for the ETF Buyout.
https://promo.sprint.com/Registration/join2015Landing