I definatly buy the book more often than not. Chances are, I reread books a few times, or like to re-read sections. This way, i always have them to read again. Parting with books is actually hard...LOL! My mom still has some of my harry potter books, and its bothering me that they aren't here....
You know, it's interesting how many people say they buy them because they want to reread them. I don't have the extra money to buy many books, but I have no problem borrowing them later to read a second time.
Since my mom works at a library, I get to borrow as much as I want. And since I have no money to buy books, well... it doesn't take a wizard to divine what I do more often.
I usually buy mine from Half.com, used. Also frequent used book stores, yard sales, flea markets. Seldom buy new. I did buy a new book online the other day because an acquaintance of mine wrote it.
I borrow all my books, because most books I will have to stop reading halfway through due to an intense disliking for the book. But when I do like a book that I have borrowed from the library, I will go to a bookstore (not a used one) and buy it so the writer of the book gets some royalties for a job well done.
I never buy books. I have a good memory and always remeber what happens in a book, I only ever need to read it once. Therefore to me buying a book is a waste of money as I'm only ever going to read it once. Imagine buying a Coat/Game and only ever Wearing/Playing it once, there's no point. And I read so much I wouldn't be able to afford every book I read. I nearly live in my library.