Forum posting for SEO is no longer as effective as it used to be. However to generate exposure and interest in your product or service it can be effective. Participating in forums can raise your profile and establish you as an authority (as long as you contribute correctly), and help you to develop your websites brand, so to speak. It can generate targeted traffic which should be your ultimate aim. Generally any link that can be obtained easily such as forum signatures etc has been devalued by Google.