The low weight/portability.
I live in NYC and I carry it everywhere I go. I think nothing of flipping it open to jot down a few lines, to check something, to reply to email... in the park, on the subway, in line at the drugstore even.
Until you've experienced how light it is day to day, you can't really understand what a great experience it is.
Peter
http://www.FlashlightWorthyBooks.com
Books so good, they'll keep you up past your bed time.