Yes I frequently sign them. I do not sign every one, only the causes I believe in. I like that when I sign a petition I often receive feedback about how successful or not, the capaign was. I am surprised that it seems to effect change. It is an easy way to make a difference as I cannot afford to donate money to so many worthy causes. I used to write letters for Amnesty International- and I think that petitions and letter writting are effective.
Haven't signed any thus far. I deem the cause is worthy enough I would sign one. Generally however, i would be flippant about affixing my signature or identity to any random cause. I feel that I could be targeted one way or another. As I have seen with persons who make comments or Facebook or twitter on certain issues.
It woukd have to be something that really needed my online signature before I would even consider it and for the same reason as some of the others, you could be targeted by advertisers and that contributes to more spam in the inbox.