Personally I think the value in signatures is directly related to your involvement in the forum itself. If you post quality replies (most people don't) then this is part of your credibility and after a few reads, people will check out your signature links to look at who you are and what you do.
I also think that
PR from forums is quite diluted, but is worth having in the mix.
Bottom line, think humans - create worthwhile responses and people will naturally check you out.