For me, it was necessity.
I used to have a pretty large collection of paper books, but I recently emigrated to America (from England) and thus had to move house by 'plane...
Nearly 700 CD's worth of albums on my ipod, over 500 movies on a hard drive, and several thousand books on my reader.
A few years ago, I would have had to pay thousands to transport my stuff overseas (or more likely leave most of it behind) but thankfully, technology allowed me to pop it all into my carry on luggage
I managed to find e-versions of all but five of my many many books. I sold all of my paper books, used the money to buy my PRS 505 and haven't looked back since.
I have now come to enjoy reading on my reader more than reading paper books, and I have no clutter.
With the e-ink display and battery life measured in weeks, my ebook reader is just as comfortable to read and convenient as reading a paperback, yet it stores a whole library.