I'm not one of those people. I hate spoilers. A lot.
A lot of my friends don't understand, so I'll explain. I'm going to use Buffy the Vampire Slayer as an example.
I started watching it recently (a friend's been lending me the DVDs), and everyone told me that series 3 and 5 were the best. So I got through series 1 and 2 and really, really enjoyed them. I was told that they weren't the best of the 7 seasons, but they were still awesome.
Then up came a spoiler. Well, more than one. At college, people started to tell me everything about series 5, 6 and 7, assuming I wouldn't care because the show's been out for nearly a decade. Unfortunately, I do care. I got to series 5, which everyone says is the best, and I realised that I wasn't enjoying it as much as the first two, even though it was supposed to be better. I'm pretty sure the reason for this was because my friends had spoiled every single detail of every plot twist and development that was going to happen.
In conclusion to that little anecdote, spoilers ruin tension. They ruin any kind of suspense. If you know what's going to happen, why watch or read it?
People have been spoiling stuff for years - from Harry Potter and the Half-Blood Prince to Charlotte's Web (I kid you not, when I was little, there was this kid at my school who took particular delight in annoying me. One day, he saw I was reading Charlotte's Web...) and I'm fed up of. Not just of the spoilers themselves, but of the nonchalent way in which people spoil things. They assume that nobody minds having something they're reading ruined.