Generally on my Nook I've gotten free or very low priced classics (like $1 or $2 for 25 books) or books on sale I wouldn't have bought otherwise. Also I download anything available on B&N's free Friday. If I'm going to spend $5 or more on a book, I want something that I can either sell, give away or keep forever. Nook and Kindle books don't meet the first two and are questionable on the last. Also my first generation nook isn't quite as readable as a book. I don't know if it my eyes or just the screen not being as high resolution as paper, but I find I can read paper a lot faster than a screen.
And for those who like stuff free, http://hunt4freebies.com/20-free-nook-books/ lists 20 of B&N's Classics series for free. Ten are free now anyway, but ten are special "kickstart" editions which look the same as the normal version, but have a different UPC and don't show up in B&N searches.