I'm looking at this for my son's birthday. He's currently using the iPhone 5 I bought four years ago on AT&T, but now it's on Cricket Wireless. He wants more memory and $250 less for the iPhone SE than the 6s is easier on the wallet. Although I see I get the company discount on the iPhone 6, but maybe by his birthday I'll get it on the SE as well. Of course, any time he wants to use his money for the iPhone, that's his call.
There are still people at work who have iPhone 4 and iPhone 4s and the SE would be a good move. You're getting the 12 Megapixel camera from the iPhone 6s so that would be an awesome upgrade from a 4/4s.