@ Well, pretty much everybody
I've had a PRS505 ever since they went on sale in the UK, being both a book buff (whatever one of those is) and a lover of gadgetry in all its forms.
Several points spring to mind:
1) As has been said, when travelling, one PRS505 is way more portable than even a couple of paperbacks. Compared to the 5 or 6 paperbacks I used to carry, it's no contest, and the electronic version allows even more variety. Battery life is good for 2 week trips without a recharge, even when spending hours every evening in hotel rooms reading.
2) The price. I paid £200 for mine. However, I have read well over 100 free books on it, there's a huge amount of copyright free classic stuff out there for free download. As these are all out of copyright, they would normally retail at £2-3 in paperback, so the device has paid for itself and more, and has expended my reading as a bonus.
3) The scope. There's a load of stuff out there for download that is out of print, or never made it into print in the first place. I've found loads of stuff from the golden age of Science Fiction, for example, all out of copyright and freely available. One day I might buy a book for download, but I doubt it will happen any time soon.
The lack of depreciation despite there being no stock shortage should be the clue. These are deeply gorgeous machines (let down, as ever, by the shite supporting software - pull your finger out, Sony....).