It is not a physical discomfort for me to read on my Kindle. But it can be for me to read a book, depending on its size. Besides, I cant adjust the print size on a book, I can on my Kindle, thereby making it easier.Reading any serious book on this device should be as uncomfortable as on any e-book reader available today, or possibly worse.
Actual book vs. e-book is totally different from MP3 vs. CD or those other arguments. Unlike when you listen to MP3 files, you can't fool yourself into thinking that you are doing fine since it's actual physical discomfort instead of some subtle quality drop.