For me it was multiple reasons like many here.
1) Got tired of my house looking like a library
2) Baen Books and a laptop
3) O'Reilly Safari books - Online Tech books.
But what pushed me over the edge and made me finally break down and get a reader was all the Indy books. I have to say that Indy books have revitalized midlist authors and I'm loving it. Now days it is rare for me to purchase a book from the big 6 much less from some of the smaller publishers. I set my price threshold to what Baen charges for a ebook ($6) which I think is a reasonable amount. Anything over that and well it better be outstanding and from someone I have read before.