I've done it a few times and it never really spoiled it for me since what I read those times wasn't that spoilery. If I had got majorly spoiled I would've been pissed at myself so I never do it now even if the book sucks. At that point I don't care about the ending at all. I do reread books quite a lot except lately. I've got a huge to read pile right now so I've no time to reread anything. Hell, I've barely got time to read new stuff now.