It helped me get ranked number one for Facebook Hacks as a keyword when I owned the site facebook hacks dot org. It was 90% of my back links. Make sure the backlink from the sig comes from a related page, or at least it helps out a lot.
overtime it will make some impact (on backlinks) but if you are a regular poster on the forums then other members will go to your site just because they see the link. I'm new to digital point and only have a few posts so far but 5% of my daily visitors are coming from here. When I have more posts more visitors will come. Of course it will take time to increase that number (bc its not my main focus, SERPS are). It can only help your website grow so definitely consider it.