I think better than signature links are links in thread, added as reference for reply.

Sample: a forum user is looking for a specific content/service. If you gonna post as a reply with a link from your site you gonna get much more visitors.

Also, if you gonna reply in a dedicated subforum (related to branch of activity of your site) and you have your site as a link in signature with proper keywords over text you gonna have visitors also.

So i think is vital to "target" your efforts with any forum signatures/forum replies so you maximize number of visitors coming from those links.