My reasons was changing from a netbook that overheated all the time to a small 5" ereader with Portability, Font sizing, Easy on the eyes and SD Card capability. Once Pearl came out I upgraded to a Kindle 3.
My main reason was I don't like used books as who knows what's on those old pages or where they been in like a bathroom.
I have bought some used books that have made me sick. I am very sensitive to things like mold, dust and pollen and a ereader is nice and clean. I still buy new paperbacks here and there but I like not having to wait for a book to ship to me as I like that instant gratification buying a ebook can give.
I was skeptical that E-Ink could emulate a paperback but I think it's better then a paperback and I love I can zoom in as needed. Try doing that with a mass market paperback that uses 10pt fonts. lol