On the 4:15pm PHL>DTW last Sunday (1.17) an older gentleman in either 2C or 3C (I forget what row I was in) had his Kindle on during takeoff, and had it on again during our approach. When the FA asked him to turn it off, he went on a ramble dissertation that it is not a computer and that he has flown to Beijing and Tokyo on Delta and has NEVER been asked to turn it off. This went on for 5 minutes and involved a second FA. He finally relented...until the FAs took their seats. Then he powered it back on.