I have had a Kindle for 3 years now and love it because it is so easy to use. I have become less fussy about what I read as I only download free books. I now have over 700 unread books. Of course not all of the free books are good, but I may have been lucky to find that most of them are quire readable. I do now only download ones that have received lots of good reviews from other readers.
What I do miss is being able to borrow books from the library, so if I was buying an e-reader now, I would consider getting one specifically for this purpose. Admittedly the library does not have an enormous number of e-books so in practise it may be a bit frustrating to use.
I have never been one for buying books so cannot comment on which system is best, or cheapest, for doing this, but certainly Amazon makes it very easy to download books.