The catch is you're under contract with them for two more years. But if you were going to do that anyway and you have an iphone 4 laying around it's a great deal. I just did it today and I didnt even buy my iPhone 6 Plus directly from Verizon, lots of missinformation going around that you have to order the phone from verizon to qualify.
Actually, you can get an iPhone 6 off contract. I am getting $250 for my iPhone 5, not going on a contract.