I think you two above posters are wrong
Forum signatures are bad practice according to google: https://support.google.com/webmasters/answer/66356
Quoting from that webpage:
"Here are a few common examples of unnatural links that may violate our guidelines:
Forum comments with optimized links in the post or signature, for example:
Thanks, that’s great info!
- Paulpaul’s pizza san diego pizza best pizza san diego
so i think it is questionable if it can improve or hurt ranking. Certainly it can bring traffic. Some signature links like on this forum are with nofollow attribute which should be safe and google should not add any value to such links, it can only serve as an advertisement to a visitors of this forum.