It honestly depends on the game for me. I refuse to watch any more trailers about Metal Gear Solid because Kojima simply gives too much away, even if the does in fact create a couple clever red herrings. With MGS4 we knew every location before the game came out, every character who would return, and pretty much the framework of the entire story.

But when I'm looking up for a new series I'm about to try like Assassin's Creed or F.E.A.R., I really won't mind if I see a spoiler or two. If I end up really liking it then my feelings my change for when the next one is coming out.