I think it is still helpful especially if you want to gain more visitors to your website, it is possible if you are not spamming the forums because if you are obvious spammer then no one will visit you sig links.
I try hard to leave intersting posts that are 100% relevant to the discussion and never start subjects that are obviously for the purpose of chalking up another post. But I always want "Value" for the information that I leave behind. The value I get is either a little bit of SEO benifit from key anchor text links (and these littles do keep adding up) I can also get relevant traffic back to the site that converts as well. SO I always keep to the one subject which is relevant to my business, telemarketing. This is my first post so I hope that I will get the signature soon and gain a little value each time.
I would most definitely say that it works great. If you are a new site or an established one, it will pay off.