Due to certain factors a forum signature link may or may not provide an SEO benefit. On most forums that allow signature links the members almost always utilize this feature. Due to that simple fact the SEO and Pagerank value of the backlink isn't that great because the more backlinks that appear on a web page the less weight each backlink will get. It is still a good way to promote your website and increase backlinks; you will also receive some traffic from the forum itself depending on the forum niche and your websites content.

Now, let's say the forum uses the rel=nofollow tag for all signature links, which means you receive zero SEO or Pagerank value from the backlink. You still would receive traffic from the forum by members visiting your site via your signature. So in all aspect even if the forum(s) you go on use the rel=nofollow tag you can still receive some benefit from the link in the form of traffic.

So, after reviewing the SEO value or non-value of a signature link it's still wise to utilize them where ever possible. Not only will you receive some sort of traffic, the forum itself in the future might remove the rel=nofollow tag from their signatures. Always make sure to use the proper anchor text in your signature backlinks. Proper anchor text would be either the keyword or keyword phrase you are trying to rank for on the search engines.