They're piss easy. We sorted out my Grandma with one and she doesn't have any problems.
The only thing we had to do with her was basically get it all set up via an Amazon account on her laptop. She found it considerably easier than fiddling about with the little keyboard or d-pad on the Kindle itself. So she can shop for books, read reviews and then do the actual buying on the laptop and the book just appears on her Kindle. Sorted.