Can't speak for anyone else, but I haven't been given any solid reasons to discredit things Snowden has brought to light, except by parties whose best interest is to undercut him. He may have got things wrong on a small or large scale and could have deliberately falsified information, but the evidence to suggest this is currently lacking. The NSA, on the other hand, has been shown to operate with questionable ethics repeatedly. For the moment, I'm inclined to give him the benefit of the doubt over them.