My personal experience has been that I used an anchor text link in my signature, in a related niche forum with a high page rank for several months, and suddenly I am getting loads of traffic from Google for that key word phrase.
The signature link seemed to work just fine for helping move my site to the forth position in the SERPS in a competitive niche.
I once heard that a man with an experience is never at the mercy of a man with a argument.
I guess
SEO is more than just a few signature links in a high page rank forum. In any case I must have done something right because my traffic has tripled.