They're completely worthless for SEO as 95% of forums rel="nofollow" the links so search engines ignore them.

They can bring in some traffic but even as one forums "tech nerd" I still only get about 4 people clicking my sig link on that forum a month so they are okay for publicity.

Just make sure the forum admin allows you to have a link in your sig first and don't spam just for the sake of having your sig displayed.