No free books, but you probably can side load books in other formats.
They are lighter than a book (especially a hardback), have similar appearance to paper, can adjust the text size.
Books cost around 20% less than the paper equivalent. In the US they have just launched a lending library at a cost of £50 a year, hopefully that will come to the UK.
Overall - yes they are worth it, my father in law at the age of 90 has just purchased an Acer tablet so he can read books using the kindle app on the backlit screen.