I have 2 sony 505 (silver and red) and 1 sony touch. I intend to keep all mine.
I have loads of books for the sonys and am not fussed about converting them for the Kindle.
I use them all and load different genres on each one, so they are in fact like a library.
When I want Fantasy, Horror and Sci fi I go for my Sony Touch
when I want Urban fantasy and all things supernatural I go for my red 505
classics, fiction and more sedate genres go on the silver 505 .
This way I get all of my ebooks onto a reader without resorting to using sd cards. I have them lined up next to my bed like books and know which genres are on which reader, and choose accordingly. Works for me.
My Kindle will store just books bought from Amazon, and will sit next to the other readers so I get a full choice of ebooks all loaded and ready to go.
I am a really bad hoarder of books and buy way more than I will ever read, but its nice to know they are all there when wanted and my poor bookshelves get a rest.