When reading I've always underlined sentences that make me happy. Once the kids got old enough to understand there's no email or fun on a Kindle I switched from dead tree books, and now the underlining is stored in Amazon's datacenters.
After a few years of highlighting on Kindle I started to wonder if the number of sentences that I liked and the eventual five-star scale rating I gave a book had any correlation. Amazon owns Goodreads and Kindle services sync data into Goodreads, but unfortunately highlight data isn't available through any API.