I'm not a parent, but I think I am going to go against the grain here.
I don't really think that these types of sites are really going to increase vigilantism. How many of you folks saw people that you knew or are in your neighborhood. Anyone of you go out and beat them up? I understand the argument, but most convictions are of public record anyway. I'd imagine that if you are determined enough to go out and rid the world of sexual predators, you'd be willing to do a bit of research about it instead of just waiting for an easy-to-use website to pop up.
I suppose I understand the argument that there isn't much you can do differently even knowing the information, but here is an example.
If I had a younger son (say 9-10 years old) and
This Guy lived next door and offered to babysit my child, I would know to avoid putting my child in danger. Granted, simply because you had done something in the past doesn't necessarily mean that you'll do it again, but I would feel a helluva lot more comfortable knowing that my babysitter has NEVER been convicted of a sexual offence with a minor than someone who had, but served their jail time or whatever.
Of course there are exceptions to every rule, and unfortunately there are people out there who -at least in my mind- don't "deserve" to be labeled as such (like a 17 year old with a 16 year old) but the fact of the matter is - they broke the law. I'm not trying to insinuate that the law is just or acceptable, but if people think it is unfair - get the law changed. I don't think that allowing people that were convicted of sex crimes (especially with children) should be allowed to live wherever they please without parents at least having the option of finding out about it...