I thought what the OP was asking is how much the actual visual words in your signature will help you for SERPs of the same words? I could be wrong. I'm also interested in this information
They help - with traffic, and getting links. And especially if its on related forums. It doesn't hurt to have your site link in all the forums that you post on, it'll do more hep than harm.
If a forum is dofollow then it will help in your ranking. If they are nofollow, leaving a comment will still provide exposure to your site and drive traffic. It will be also useful in creating networks with other members.
Putting your links in your signature can increase your traffic and can help also to push your keywords to rank in search engine.
Depends on the forum. I look and participate in several. This place provides very little traffic AND very little evidence in webmaster tools for inbound links, provided the number of posts put up. No Follow, do follow, doesn't matter. Not sure why Google hates DP sigs.
WE definitely get some benifits from it, But how much is totally depend on that forum site. A webpage give benefits to pagerank is pagerank of that website divide by number of outbound links on that page.