Last year I "traded in" a couple of iPhone 4s through them. Unlike my previous experience with their competitors, their service was terrific and they played no tricks on me with valuation once they received our devices. Watch out for that-- you might be quoted an impressive $XXX for a device from some less scrupulous company but end up getting significantly less.
Wow. It seems I got the most for my iPhone. I sold my 16GB iPhone 5S (gold) for $520 after being talked down from $530 for cosmetic damage my buyer didn't see in the 16MP images I posted. If you ever think you'll sell your iPhone, keep the box and as much in it as you can.