I have many times, but I found them to be real issues. Mostly with the iPhone 4/5 generation and yellow screen and the 6 generation mechanical/button/camera problems. 6S has been completely flawless. Yellow screen is #1 on the list, it isn't placebo, it doesn't go away, it isn't "glue" it is the WORST. a very yellow screen literally ruins the phone. It is terrible. Turn on night mode on your phone with a normal screen, then imagine that all the time. That is exactly what it is like. That being said, I think the 6S generation has been the best quality wise they have ever produced. Brilliant display, great mechanical quality, and solid overall build. Most of the time the issues I think were cascading in nature - the replacement / refurb devices were of much lower quality or different components, so the replacements would basically have many more problems than the brand new phones. Found this to be pretty true.