At first I bought the Sony because it was the only one available retail. Then I later upgraded to the Kindle because I wanted Text to Speech and multi-lingual dictionary support and it was the only reader which had both of those features.
Somebody like my mother, she is not entirely convinced she needs/wants one, but I have offered her one of my extra ones and her main concerns are that she can put things on it by herself without my help and that she can carry around about 10 books at a time with her. And really, any device out there now will meet those needs
She later thought about this some more and clarified that 10 books will be here MINIMUM and that in an special situation like if she was going to Florida for a month, she might want to carry around 'more like 12 or maybe 15' books...