Verizon's offer of a free iPhone 6 to anyone who turns in an old iPhone and signs a two-year contract is a good deal. The iPhone 5s is selling for around $99 so its worth half that used. Verizon is going to give you $200 to purchase a new iPhone 6, no matter how old your iPhone is. That's a great offer.
It is a good deal if you want t go with Verizon and you appreciate their service. Their prices are often higher for the contracts, but they are often the best when it comes to calls and 4G. However, if they are offering the phone for free it's because the contract makes them a lot more money and that should be considered.
Verizon offers "free" phone deals in order to lock new clients into their contracts. While signing up for a contract would be cheaper than the real cost of an iPhone, the monthly cost of the contract for talk, text and data usage will quickly catch up to and surpass the cost of the regularly priced iPhone.
Of course giving away a free iPhone 6 sounds like a great idea to the respected customer, but it sounds like a bad idea for Verizon as a business. I am sure they can afford to give away the iPhones as a deal with the contract, but that's still a lot of money and revenue they will be losing.